There isn't really a good way to do this. The "correct" way would be to delete that group from staging, and then migrate it, but in this particular situation that's very rarely the acceptable way to do it.
As of Integrity Lifecycle Manager 10.5, there is a way to do this, but it involves making all groups and users editable on the production server, and should be used with caution. See CS91512.
I hope that helps,
Kind Regards, Kael Lizak
Senior Technical Support Engineer PTC Integrity Lifecycle Manager