Adding wordpress to existing environment

Is it possible to add the wordpress platform to an existing environment so that the use would not need to create a new login just for wordpress.

Basically, I have a client who wants to use a CMS in order to pass new content out to their dealers. The dealers already utilize a firewalled site to get some of their information. How can I go about adding the wordpress platform to that site/environment and have it use the dealers’ existing login information to access the wordpress portion of the site?

I’ve been talking with the folks over at MovableType who claim that they can do this (for a consulting fee). I’d rather not go through them, though, due to some issues with my client’s lawyer.