Change the USERS database

I have a simple question:

Which files do I have to edit to get the users from another database then my wordpress uses?

What I want is this:

I have a userdatabase for my discussionboards (XMB_) and a separate userdatabase for my wordpress (WP_) on a different location. I want to integrate the XMB_ database into wordpress so that users are automaticaly logged in to wordpress as when they logged into the forums.