PhpBB is open source as well. Don't give up man, just do some searching on the net. It took me a while to find something on phpBB. You can look at their code and see how they authenticate - it will be difficult though. You could also write your authetication info - account, user, pass, email, etc to the forum database using your own scripts, but again, you will need to dig into their code ands structure to figure out how they have it designed.
Good luck man!