Course' there's one!
However It's utterly complicated to be able to play it. In order to play the whole game in Challenge Mode, you must first delete your previous save file permanently (UP+SELECT+B) on the title screen and ask someone else with a B2 Copy to transfer the challenge key to your current W2 game (with the save deleted). If you only wanted to play the postgame in Challenge Mode however, you don't need to delete your save file - You'd only need someone to transfer the key to your game.
In B2, you will gain access to the Challenge Mode Key after defeating the champion, however in W2 you will obtain the Easy Mode Key instead.
Gamefreak at its finest. Good luck with that, and hope this helps!
Source (All Gameplay Modes in BW2)
About Save File Deletion and Key Transferring Needed