You could probably use Nathmatt's Individual Enemy Kill Counter or Game_guy's Kill Counter to keep track of how many enemies the player has killed, and then have a parallel process event that will add more enemies when the kill count gets to certain points. The difference between the two scripts is that Game_guy's counts all enemies together and Nathmatt's counts individual enemies. Since you want zombies that get harder to kill, you probably want to make each difficulty level of zombie a different enemy, and therefore you would want to use Game_guy's script.
I've never played Zombie mode in Call of Duty so I don't understand what you mean by unlocking new rooms. Are there locked doors that must be opened with keys or after you've killed X amount of zombies or must be unlocked with achievements? If it is one of these, there is no reason to make an entirely new map for each room. Just have a door that is locked and opens if player has key/X kills/unlocked achievement. Also you would want to disable any zombie spawning points in those rooms when they are locked.
I also don't understand what you mean by different maps. Do you want multiple different maps that the player can choose from? If so, you can have a "Map Selection" room where the player can choose which map to warp to. You can also have different maps there that must be unlocked by disabling their warps. Or do you mean that the player can design their own maps in-game? There might be a script for that but I doubt it (it would be really cool). I think there is a script for a random dungeon generator that might work if you want random maps.
I hope this helps.