Hi @sazzer ,
This answer might be what you are looking for: Signup with username & social connections - #2 by Abhishek_Hingnikar
As described in the answer, you could set up a redirect rule for social logins to collect the username field on a special page: Redirect Users from Within Rules
User signs up with social connection, after checking email and email_verified . You can check if the user has a connected identity with a database user in user.identities array. If such an identity exists, the rule will pass through if not, the rule will redirect the user to a special page which asks the user for their username and a password.
On this page, you can setup your API server so that it accepts, a username-password pair from this page and creates a secondary identity for that user using Management API v2 . The page will then redirect the user back to the /continue endpoint and when #1> occurs it lets the user pass through.