I'm sure that there is a way to fully an safely synchronize information between your two sources. Until you find that out here is a work around that will also provide you a mechanism for backup in case something happened to your registered users. Export your new registrants to a CSV file. I use WP-export and it has the capability to schedule an automated export of your registrants . As you develop your staging site you can import your new users into the side as you need them.