i partially managed to do this by managing to have 2 parts in my site - the forum and the normal pages. i have a password-protected area which has a login page nicely written. when users log into this part, it will log them into phpBB as well when their username and password is identical. if the user is already logged in, nothing will happen and it will appear to seem that the user had just logged into the password protected area.
I dont know if this is related, but to me it seems so thats why i posted this lol