Converting over facebook logins from custom site to wordpress social

This maybe technically a buddypress question. I have 10,000+ users that I’m moving over from a legacy custom site. I think about 5000 of them authenticated to the custom site through Facebook. So we have tokens associated with them. Is it possible to reuse/transfer those tokens or what we stored to move to a social login through buddypress or an WPMU social related plugin? Otherwise I don’t know how these users would login to the new WordPress site with Buddypress and WPMU plugins.