There are so many lame attempts to integrate a forum into PHPmotion. Most have failed because they take over the site and make the forum the focus. Other fail because the massive security with PHPmotion.

Would there be a way to integrate UPB into PHPmotion using PHPmotion's login, registration, message and user profile system instead of the UPB system?

I am actually looks at a few forum options to solve this.

short answer: no.

long answer: I suppose it would be possible, but unfortunately while UPB2 is much more modular than previous versions, it isnt abstract enough with the way it handles users to be shoehorned into that kind of system (at least as best I can tell going by a quick browse through the PHPMotion site). Not to mention that UPB uses flat-text files for its databases, while PHPmotion uses MySQL, which would bring in an absolute mountain of compatibility problems.

Realistically, if there is already other forums that offer some level of compatibility I would recommend that you start there.