I don't have a solution recipe for this but you would essentially be adding commerce to your registration site. The commerce would be handled by using eCart. The membership will be the item. Once you add the membership to the cart you can then send them to the checkout page directly to pay for the membership.
You will need to update your users table to have a column to indicate weather or not the membership has been paid for. Once you have this column you will just need to update it for the current user when the checkout comes back successfully.
The recurring billing is not a supported feature of eCart though, so you would need to deal with this in another way.