Probbaly would need to do some customized hacking to get that done.
The addon basically ads them to a group when they make payment for x amount of time. You probably could hack so that the group gets changed after x posts in that forum. But the integration would probably need to be pretty deep.
The paypal thing is really designed to add them to a group for x number of days, then removing them when they cancel or the time period has expired.
