The game is already made! They can't add something like this.
You think it matters whether or not the game is made, you can download entire games online now. Why wouldn't you be able to do something like this? Also it would be simple compared to what other games have. Halo 3 has the most extensive map maker that I have ever seen and a zombie map maker wouldn't need that many features.
start complaining to treyarch that the console users get left out
don't think it will work though
you could try another technique that will most definetly work. just say that you'll pay for the map maker function since all they care about is money