Are you looking to have one user database for your entire site as so the user's do not have to log in more then once for different parts of your web site?
UBB does not currently support that as far as I know. This would have to be a hack in then login process of the UBB. The problem is that every time you make an upgrade to a newer version of the UBB you will have to re do your hack again. The places you may need to add your code may not be in the same exact files and some variables may change form one version to the next as well.
