DataMesh - DF User Profile with virtual foreign key

I have a legacy application which has a Users table (with user account credentials). I would like to use the DF DataMesh feature to establish a virtual foreign key between this table and the DF User Profile. I believe I must first create a locked-down Service connection to the DF backend, but I cannot get the connection string right. What database name and credential set (root?) must be used? Is this approach even possible? Alternatively, can the User/Profile service exposed via API Docs be customized to include a relation or virtual foreign key to an external service? Thanks for any advice you might have.

The User service cannot have virtual keys associated with it since it is not a database service. It is a user model that itself is built on a database with multiple relationships.

My best advise is to give users lookup keys ( ) whose values correspond to some unique value in your existing user table

Then when you access the user model you can have event scripts that also pull data from your legacy user table, using the the lookup key as an advanced filter on the role.