With the Stadium Unlock plugin, it becomes possible to unlock the Stadium environment mixing in ManiaPlanet. This is normally intentionally blocked by Nadeo for several reasons, but technically everything works just fine.
First, make sure you enable the patch in the scripts menu:
Then, in a multi-environment title pack (such as TMAll@domino54), you can simply create a new map like normal and select any other vehicle.
Note that when you save and load such maps, you will still need to enable the Stadium Unlock patch in order to play these maps again! If you don't enable the patch and load a map, you will spawn in with the regular Stadium car.