Adding users through backend

I’ve search and searched without much luck, but was curious as to whether there have been discussions/insight as to the success of people adding users to WordPress through a backend process. In my case, I have an existing site, that has extended user attributes, including groups, that I would like to maintain while still proving integration into WordPress. I could still keep them separate, as the integration isn’t necessarily required, but a single sign-on is definitely desired. I’ve found the HTTP authentication plugin that could perhaps meet the needs of single sign-on, but the users would still need to be added via the WordPress gui.

So the question is, has anyone developed their own bridge to add users to WordPress, other then SMF, phpBB, etc..? I’m sure I could reverse engineer the DB to accomplish this, but was hoping to leverage the understanding of others first.

Thanks for any insight you can provide!