Joseph Smarrs Rezept, wie man OpenID nachträglich in eine bestehende Community integriert:

This is a step-by-step tutorial guide for implementing OpenID consumer-side support with a web site that already has users with accounts. It will explain how to easily let new users sign up for an account on your site using their OpenID URL and how to let existing users attach their OpenID(s) so they can sign in using them.