DotNetnuke uses AspNetSqlMembershipProvider by default, so the tables are the ones that have: aspnet_
The password is encrypted (usually), but you 'may' be able to point another site to those same tables if you set it up to use AspNetSqlMembershipProvider.
Another option would be to write your own Authentication provider in DNN that checks the other website's username/password instead of using AspNetSqlMembershipProvider.
Google: DotNetNuke custom Authentication Provider.