Use car from different class:
To be able to use a car from one class while racing in another class you have to start your game with the game course type that you want to race in.

Then go to the Game Settings screen and select the class of car you want to use. Then press B to exit back to the lobby and go to Car Selection screen and you will then be able to choose a different class of car from the race you are in.

This can only be done online and you must be the host of the game.