Okay, this makes it easier to talk about it.
There should be something in the drupal code that copies a user registration to ubb.threads. You can have a look in /scripts/adduser.inc.php what code is needed to add a user to ubb.threads.
Now you would also want to create a logon cookie for the user, so that they don't have to log in. /admin/loginas.php could be of help on that field.