diff --git a/back/admin/integrations/models.py b/back/admin/integrations/models.py
index a14489411..5f62a0308 100644
--- a/back/admin/integrations/models.py
+++ b/back/admin/integrations/models.py
@@ -220,6 +220,13 @@ def is_sync_users_integration(self):
     def can_revoke_access(self):
         return len(self.manifest.get("revoke", []))
 
+    @property
+    def can_backfill_ids(self):
+        # True when the manifest's `exists` block declares `store_data`,
+        # meaning we can extract IDs from the lookup response and backfill
+        # them into existing users' extra_fields.
+        return bool(self.manifest.get("exists", {}).get("store_data"))
+
     @property
     def update_url(self):
         return reverse("integrations:update", args=[self.id])
@@ -518,6 +525,7 @@ def user_exists(self, new_hire, save_result=True):
 
         self.new_hire = new_hire
         self.has_user_context = new_hire is not None
+        self.params = new_hire.extra_fields
 
         # Renew token if necessary
         if not self.renew_key():
@@ -530,6 +538,28 @@ def user_exists(self, new_hire, save_result=True):
 
         user_exists = self.tracker.steps.last().found_expected
 
+        # If the user was found and the manifest declares store_data on its
+        # exists block, capture those values into extra_fields. Lets a single
+        # lookup populate IDs (e.g. ATLASSIAN_USER_ID, bitwarden_id) for users
+        # that pre-existed in the upstream system.
+        store_data = self.manifest["exists"].get("store_data", {})
+        if user_exists and store_data:
+            try:
+                json_response = response.json()
+            except (ValueError, AttributeError):
+                json_response = {}
+            for new_hire_prop, notation in store_data.items():
+                try:
+                    value = get_value_from_notation(
+                        self._replace_vars(notation), json_response
+                    )
+                except KeyError:
+                    continue
+                if value is None:
+                    continue
+                new_hire.extra_fields[new_hire_prop] = value
+            new_hire.save()
+
         if save_result:
             IntegrationUser.objects.update_or_create(
                 integration=self, user=new_hire, defaults={"revoked": not user_exists}
diff --git a/back/admin/integrations/serializers.py b/back/admin/integrations/serializers.py
index 12130b926..89e0d0d84 100644
--- a/back/admin/integrations/serializers.py
+++ b/back/admin/integrations/serializers.py
@@ -51,6 +51,7 @@ class ManifestPollingSerializer(ValidateMixin, serializers.Serializer):
 class ManifestExistSerializer(ValidateMixin, serializers.Serializer):
     url = serializers.CharField()
     expected = serializers.CharField()
+    store_data = serializers.DictField(child=serializers.CharField(), default=dict)
     status_code = serializers.ListField(
         child=serializers.IntegerField(), required=False
     )
diff --git a/back/admin/integrations/tasks.py b/back/admin/integrations/tasks.py
index bcb9776ba..4de43fa32 100644
--- a/back/admin/integrations/tasks.py
+++ b/back/admin/integrations/tasks.py
@@ -1,8 +1,12 @@
+import logging
+
 from django.contrib.auth import get_user_model
 
 from admin.integrations.models import Integration
 from admin.integrations.sync_userinfo import SyncUsers
 
+logger = logging.getLogger(__name__)
+
 
 def retry_integration(new_hire_id, integration_id, params):
     integration = Integration.objects.get(id=integration_id)
@@ -15,3 +19,50 @@ def sync_user_info(integration_id):
     # users or we will add new users. This is done in the background.
     integration = Integration.objects.get(id=integration_id)
     SyncUsers(integration).run()
+
+
+def backfill_integration_ids(integration_id):
+    # Run the integration's `exists` lookup against every user. Any
+    # store_data fields declared on the exists block get written to the
+    # user's extra_fields. Used to populate IDs for users who were
+    # provisioned in the external system before this integration existed.
+    integration = Integration.objects.get(id=integration_id)
+    store_keys = list(
+        integration.manifest.get("exists", {}).get("store_data", {}).keys()
+    )
+
+    users = get_user_model().objects.exclude(email="").order_by("id")
+    matched = skipped = not_found = errored = 0
+
+    for user in users:
+        # skip users who already have all backfill keys set
+        if store_keys and all(k in user.extra_fields for k in store_keys):
+            skipped += 1
+            continue
+        try:
+            result = integration.user_exists(user, save_result=False)
+        except Exception as e:
+            logger.warning(
+                "Backfill error for integration %s, user %s: %s",
+                integration_id, user.email, e,
+            )
+            errored += 1
+            continue
+        if result is True:
+            matched += 1
+        elif result is False:
+            not_found += 1
+        else:
+            errored += 1
+
+    logger.info(
+        "Backfill complete for integration %s: "
+        "%s matched, %s skipped, %s not found, %s errored",
+        integration_id, matched, skipped, not_found, errored,
+    )
+    return {
+        "matched": matched,
+        "skipped": skipped,
+        "not_found": not_found,
+        "errored": errored,
+    }
diff --git a/back/admin/integrations/urls.py b/back/admin/integrations/urls.py
index 8f182daee..b2117cb0c 100644
--- a/back/admin/integrations/urls.py
+++ b/back/admin/integrations/urls.py
@@ -33,6 +33,11 @@
         views.IntegrationDeleteExtraArgsView.as_view(),
         name="delete-creds",
     ),
+    path(
+        "backfill_ids/<int:pk>/",
+        views.IntegrationBackfillIDsView.as_view(),
+        name="backfill-ids",
+    ),
     path(
         "tracker/",
         views.IntegrationTrackerListView.as_view(),
diff --git a/back/admin/integrations/utils.py b/back/admin/integrations/utils.py
index c2e4daf68..89a661859 100644
--- a/back/admin/integrations/utils.py
+++ b/back/admin/integrations/utils.py
@@ -1,27 +1,72 @@
+def _tokenize_notation(notation):
+    # split on '.' but keep [...] groups intact, so values inside a filter
+    # expression (which may themselves contain '.', e.g. emails) aren't split
+    tokens = []
+    buf = ""
+    depth = 0
+    for ch in notation:
+        if ch == "[":
+            depth += 1
+            buf += ch
+        elif ch == "]":
+            depth -= 1
+            buf += ch
+        elif ch == "." and depth == 0:
+            if buf:
+                tokens.append(buf)
+                buf = ""
+        else:
+            buf += ch
+    if buf:
+        tokens.append(buf)
+    return tokens
+
+
 def get_value_from_notation(notation, value):
     # if we don't need to go into props, then just return the value
     if notation == "":
         return value
 
-    notations = notation.split(".")
-    for notation in notations:
+    for token in _tokenize_notation(notation):
+        # filter form: optional_key[field=expected] - pick first list entry
+        # whose `field` equals `expected`. Useful when the upstream API returns
+        # an unfiltered list (e.g. Bitwarden /public/members).
+        if "[" in token and token.endswith("]"):
+            list_key, _, filter_expr = token.partition("[")
+            filter_expr = filter_expr[:-1]
+
+            if list_key:
+                try:
+                    value = value[list_key]
+                except (KeyError, TypeError):
+                    raise KeyError
+
+            if "=" not in filter_expr or not isinstance(value, list):
+                raise KeyError
+
+            field, _, expected = filter_expr.partition("=")
+            for item in value:
+                if isinstance(item, dict) and str(item.get(field, "")) == expected:
+                    value = item
+                    break
+            else:
+                raise KeyError
+            continue
+
         try:
-            value = value[notation]
+            value = value[token]
         except TypeError:
-            # check if array
             if not isinstance(value, list):
                 raise KeyError
 
             try:
-                index = int(notation)
+                index = int(token)
             except (TypeError, ValueError):
-                # keep errors consistent, we are only expecting a KeyError
                 raise KeyError
 
             try:
                 value = value[index]
             except (TypeError, ValueError, IndexError):
-                # keep errors consistent, we are only expecting a KeyError
                 raise KeyError
 
     return value
diff --git a/back/admin/integrations/views.py b/back/admin/integrations/views.py
index 832bfc033..bfe625fac 100644
--- a/back/admin/integrations/views.py
+++ b/back/admin/integrations/views.py
@@ -15,6 +15,7 @@
 from django.views.generic.detail import DetailView
 from django.views.generic.edit import CreateView, DeleteView, UpdateView
 from django.views.generic.list import ListView
+from django_q.tasks import async_task
 
 from users.mixins import AdminOrManagerPermMixin, AdminPermMixin
 
@@ -244,3 +245,25 @@ def get_context_data(self, **kwargs):
         }
         context["subtitle"] = _("integrations")
         return context
+
+
+class IntegrationBackfillIDsView(AdminPermMixin, View):
+    def post(self, request, pk):
+        integration = get_object_or_404(Integration, pk=pk)
+        if not integration.can_backfill_ids:
+            messages.error(
+                request,
+                _("This integration has no store_data declared on its exists block."),
+            )
+            return redirect("settings:integrations")
+        async_task(
+            "admin.integrations.tasks.backfill_integration_ids",
+            integration.id,
+            task_name=f"Backfill IDs: {integration.name}",
+        )
+        messages.success(
+            request,
+            _("Backfill started for %(name)s. Users' extra fields will populate "
+              "as the lookup runs in the background.") % {"name": integration.name},
+        )
+        return redirect("settings:integrations")
diff --git a/back/admin/settings/templates/settings_integrations.html b/back/admin/settings/templates/settings_integrations.html
index 17174d36c..bf5bf8370 100644
--- a/back/admin/settings/templates/settings_integrations.html
+++ b/back/admin/settings/templates/settings_integrations.html
@@ -65,6 +65,15 @@
               {% translate "Update credentials" %}
             </a>
           {% endif %}
+          {% if integration.can_backfill_ids %}
+            <form action="{% url 'integrations:backfill-ids' integration.id %}" method="post" style="display:inline-block"
+                  onsubmit="return confirm('{% translate "Run a lookup against every user and store any IDs returned by the API? This will call the external API once per user." %}')">
+              {% csrf_token %}
+              <button class="btn btn-primary" style="float: unset">
+                {% translate "Backfill IDs" %}
+              </button>
+            </form>
+          {% endif %}
           <a href="{% url 'integrations:update' integration.id %}" class="btn btn-primary">
             {% translate "Update manifest" %}
           </a>