If your database is of modest size I'd suggest checking out the e107 cms (www.e107.org) which integrates forums, members only pages and private messaging. There are chatroom plugins and loads of stuff. It's open source and *free*!

I think there are DB conversion scripts for some forums etc, but I'm not sure which ones.

If your database of existing members is too big, or if there is no conversion script for your forum DB then I apologise for wasting yr time.