Installing the Play as the Joker DLC and Prey in the Night DLC:

First, go to <path-to-game> and you will see many folders including BmGame and Binaries. FOR PREY IN THE DARKNESS and PLAY AS JOKER,  copy BmGame folder from my file, merge that with the game's folder of the same name, and replace as necessary (this places the files). This will place PREY IN THE NIGHT into your game. For PLAY AS JOKER, replace ShippingPC-BmGame.ex in Arkham Asylum's Binaries with the one in Binaries for STEAM or Binaries for EPICGAMES depending on the platform (this calls and activates the DLC). 


It is worth noting that if you start a new save file Play as Joker is not immediately available, as in the game you need to grab certain riddler trophies to play these maps (though you can see it is waiting for you if you click 'Challenge Mode' and should see the Joker and Batman in grey). If you want to play with them immediately, you can use one the save files from MR. JAG and convert to Epic Games in the link at the bottom of the page:

MR. JAG Arkham Asylum Saves:

https://drive.google.com/drive/u/1/folders/0B-t2fAhN2YD9eVFBeWlTTGx4eUU

OTHER WAY (MANUALLY editing ShippingPC-BmGame.exe): 


To find out how I created the new ShippingPC-BmGame.exe I just did below:
Go to offset 12153c6 (search for 25 00 00 08 00 31 86 80 in a hex editor)
    Change the contents to 0D 00 00 08 00 89 86 80 06 00 00
    Save the file.

If you have any questions about editing files including, changing save games, and more, this page is extremely helpful:

https://www.pcgamingwiki.com/wiki/Batman:_Arkham_Asylum