How do you use a in app purchase on a cracked app?

Hey! I have a cracked version of Modern Combat 2:Black Pegasus on my iPod touch because I only have 2 dollars and 50 cents in my iTunes wallet. I recently updated the app using install0s and noticed that 3 maps only costed 1.99 and I had the money for that. But it said I need to download the app first. If anyone knows how to get it, through iFile(or ssh) or any other way, I'd appreciate it. Thanks guys! Also if it made it so I'd get the maps for free, then I'd just use the money for another gameloft game so I'm still supporting them.