Modding Call of Duty games. Can you do it online and is it a good idea?

It is to my understanding that you are able to mod games such as Call of Duty. (I prefer zombies on black ops) But are you able to do it online and can you get banned for doing it in a private match, and if you can do it in a private match online how do you do it on YOUR gamertag?

Live to die.

Discuss this in "Gaming" please. Do check what you're posting in before you post.

Catch my drift.