copying database tables is apparently no option, as the order and content of the user tables will most probably differ.
The bridge files might be an option if you were running on the same server (you could create a bridge file for nuke as well as for a bbs), but it will only cause troubles if you're trying to accomplish two things at a time: creating a new birdge file is hard enough - having cpg and the community software running on two different domains (and that's the factor that matters, as browsers make a difference on cookies from different domains, not different servers) will make it much more complicated.
What I'm trying to say: this could be done (at least in theory) by someone who is a very skilled coder with in-depth knowledge on webservers and related issues. If you think you are this person: go ahead! Others (be it me or any other user on this board) will not be able to help you, as it will take hours to accomplish.
As you seem to be reluctant to post link(s) to your site(s) I'm just guessing: you're running on a tight budget and have your gallery and nuked site split up on servers featuring free webhosting (or at least very cheap hosting), and you're looking for a solution to overcome your tight budget with clever scripting that will feature a community site like those of people who actually pay for webhosting to have everything working as it should.
GauGau