integration with existing portal

As I understand it, the Member API uses the login and registration of an existing site to pass the username to WWF. What if I want to do the opposite and use WWF's system for the parent directory? I want users to be able to pass back and forth between the existing site and the forum and track the username throughout so as not to require dual logins. In addition, I need much more information in each user's profile. If I use my existing registration (and expanded profile page) in the parent directory, can I link WWF back to this profile information, or is it easy to add the additional fields I require to the profile page in WWF and maintain those existing links? I want to be able to take advantage of the Buddy List, as that will also be key in sharing some items in the parent directory. I apologize for this long post, my many questions and my apparent ignorance.