Bulk user import into M2P membership

We're developing a new WP site for a client based in their outgoing proprietary CMS. We've imported their 1400+ users to the site and are using M2P to manage permissions. Is there an easy way to item M2P to recognize the existing users without having to manually move them over one-by-one? And, can we also apply memberships to users based on a set field associated with the user? (e.g. User 1 is a "branch member" so they should be added to the "branch" membership in M2P)