Interactive 'converge'

Maybe it's already possible, if so, please tell me how :lol:

Ok, taking the idea of converge but to a slightly higher level.

For log-in authentification, be able to specify an external (interactive) method. Put in the URL of the external site's authentification URL, then a field for a user ID & password.

Then when it gets used, the external site, upon getting activated on the url with the information, returns back limited information on the account (if both the system id/pass and the user id/pass match).

Sort of the way it's done now, except that the external site does the database work so that it's password & stuff is kept localized instead of having to spread it to multiple sites.