so you want to manually register users?
yes, this can be done. in the archived documentation section of the security support page, there is a tutorial for user level authentication, it was originally created for SA1, but the principle is the same.
register your user at the admin user level, then protect the registration page using the admin user access rule. this will make the registration page available only to you, then you can register new users manually, only user you register will be able to login.