This map will be based on a alternative universe went mexico join the axis and begin large scale push intro USA

You don't need a new map for that. Any of the world maps + custom scenario does it?

I am assuming that you are relatively new to WZ. But basically, as you level up, maps and features will get unlocked. One of them is custom scenario. You don't need to have them unlocked to join games, but you need them unlocked to create/host the game.