Hello, and welcome! Sorry that you lost your chickens. Before you get any new one's make sure that nothing else will be able to break into your coop and run. If you cross breed those with the black copper marans, it will no longer be a breed, but will only be a mixed breed as that it what the resulting chicks would be. It would be ok for you to do that if you were only interested them for personal use, but you would not be able to show them.