WP Users automatically become members in the Membership plugin

I have a 12,500 user database in my WP site. We have only one member level--it's free, all we require is that visitors create an account; it has same usage/rights as WP's default Subscriber which is what all users in the DB currently are set at...so all I need is to designate the 12,500 users as being "all content" members. I do not want to do that one member at a time. So this really isn't an import question I guess as much as: is there an easy way to just put the free membership level in for every existing user? I'm thinking there must be a table in the db that I could just autopopulate via a query or some such?

Thanks.
Krista