Just display a notice to them then...
I mean I don't see why you want to go to all this trouble just due to some misdirected members. Provide them with the information that they are still on your site and that the registration processes have been merged or something like that. If you are still adamant on creating your own page then you can try:
Add new Users (automatically).
But from my point of view it is a bit of a waste of time to be honest.