Anyone Good at Hacking iPhone Apps?

I used to play a game on my iPhone, that I enjoyed, i tried using a Jailbreak on it, in order to make in app purchases for (Free) , called IAP Cracker, I understand that IAP Cracker is built on MobileSubstrate which allows developers to inject their code into any app. My guess is that it uses this capability to override the StoreKit functions in your app to return a bogus reply from the iTunes store, indicating a successful purchase, thereby tricking your app into thinking the user purchased the item. But on Some apps such as this one it doesn't work. Is there any way I can make it so that it can work? The app is called "Party in My Dorm"

Read more: Anyone Good at Hacking iPhone Apps?