I have this very bizarre situation...I have one user I cannot import into Unity. I can import all other users from the same domain without issue. There doesn't appear to be apermissions issue. I attempt to search for the user's alias, and Unity returns no results. I have checked AD to confirm that the user alias is correct. I also checked Unity to verify the user is not listed in Unity by chance.
I have no idea why Unity can't seem to find this user...
I think you can go to \CommServer\Utilities\RemoveSubscriberProperties and copy the whole folder and run the exe on a domain controller for your other domain.
It will not hurt anything anyway. If the user has a bunny head he has Unity propeties and they need to be removed. just right click on the user name.