You can import your existing WordPress users into the simple membership plugin as members. You will be able to select which membership access level a user goes into when importing the users.
Do the following steps to import WP users into the simple membership plugin
Step 1 (Install the Plugin)
Download and install the Simple Membership WP User Import addon.
Step 2 (Go to Settings Interface)
Click on the “WP User Import” menu from the simple membership plugin’s admin interface.
Step 3 (Import the Users)
You can import all of your WordPress users to Simple Membership in one go or selectively import users from this addon’s interface. You will get to choose which membership access level your imported user(s) go to and a few other details.