i've done a similar 'mod / hack' for 2 sites. 2 different approaches.
approach 1: the site uses a htaccess list, which is maintained by a 3rd party program. that 3rd party program essentially updates that list based upon credit card subscriptions.
then the ubbthreads side simply checks when a user tries to access ubbthreads.php and sees if they exist in the DB (threads db) yet. if they don't then a new threads user is created, added the the 'User' group and they then access threads.
approach 2: similar to what a portal-master, threads-slave would be setup as. as the portal verifies new user signups, does all the email verification etc and adds the user, it then sends a message to threads to 'add a new user' in a URL (get). ubbthreads checks to see if that user exists, then adds the user to the appropriate group.
threads then goes further to note the portal cookies for username and takes care to setup the corresponding ubbt cookies to allow the users to transparently log in..
sooo, it can be done... just how you want to slice / dice