Hello,
I don't believe adding a forum to an existing site would cause you to lose page rank so long as you kept spam and undesirable content off your forum.
If your site uses PHP and MySQL you can likely purchase a vBulletin 5 license from
www.vbulletin.com and install VB4 or VB5 on your site.
It would be your responsibility to customize the look of the forum (if you wish to) to make it blend in with your existing site.