flagrow/masquerade currently handles adding custom user PROFILE fields and not so much upon registration. I did mention it before to @luceos about having the option to add these fields to the registration modal but I believe it would have been more work that necessary to do.
You could however HOOK into Masquerade and add your own option to add the custom fields to the registration modal itself, but then I remembered that there was a certain bug that is being patched for beta 8 that fixes extending the user registration modal.
