You'd end up hijacking their registration code to insert the username/password they register with (along with any other info) into the ubb tables; you would probably also want to edit the login code to populate the ubb cookie to make them auto logged in...
The you'd probably want to remove the register/login links from UBB.Threads and have everything handeled via your external blog.
Last edited by Gizmo; 11/28/2007 8:51 PM.