Go to "Batman Arkham City GOTY\BmGame\Config\PC" and put this on PCGame.ini, make sure to change the Id to any number.

+PlayableCharactersV2=(BaseId=0, Id=X, Name="Batman_Joker", bBad=false, Mask=0)

If you have any trouble with this, you can watch this video: https://www.youtube.com/watch?v=nAhccA-5J4Y