If you want to play some custom map of AGAINST, and obviously, someone has already made custom maps for the game, this guide wil show you where to find them.
Did you see that Custom Maps tab in the game?
I finished the main campaign and wondered where I could find custom maps. And apparently, some map maker (or a group of map makers) for this game made a site for that (I found their DS server). And some customs levels are already available!
I hope this is helpful because at first I thought custom songs would only be available in the future.