How often would this be done? Programmatically (via API) or via other less sophisticated ways (e.g. via CSV import in bulk)?
I don't have a working lab at this moment in time - but have you tried exporting these endpoints via Context Visibility and checking if the custom attributes are included in the CSV export? If yes, then you should have a solution to filter the output in a spreadsheet (e.g. Excel Data filters) and then update the resulting endpoints with their new Endpoint Identity Group Name. And then re-import those updated endpoints.
A lot smarter would be via API - although, it would require development effort.
If you have a recent version of ISE with the Swagger UI, you can explore the API method via the ISE GUI. There appears to be support for setting the groupID (Endpoint Identity Group) in bulk API operations.
PUT /api/v1/endpoints/bulk/
The trick is to find the groupID (which is a hyphenated ID string) ahead of searching the endpoint database, and then creating a list of endpoints to update with such a bolk API call.
