Lemoon fully supports the Membership API. You can implement your own provider or use one of the standard providers, e.g. SqlMembershipProvider or ActiveDirectoryMembershipProvider.
If your provider is implemented correctly and you have configured Lemoon to use your provider you should be able to login via the admin interface. If Lemoon detects that no admin user exists for the configured provider, the first user to login should be promoted to administrator. If that doesn't work you could try setting the IsAdmin property to True in the User table.