Dedicated Master Site

We are planning to have a Multisite installation with Buddypress, and another site as local listing, and a news site for the same community.
The question is that if it's possible to have a dedicated master site just for user information like and have this 3 sites (and the future ones) as slaves.
Also can someone register from the slave site and have his information pushed to the master one so that he can login to other sites too.