If you want to do custom things in the interpreter for iOS, this is done in Xcode.
So far, I don't think anyone has tried to integrate T1 with Gamecenter. At least I haven't seen anyone say that they did.
I doubt you'd be breaking any rules doing custom things to the code provided by AGK. Unless you try to customize their stuff and sell the interpreter itself (as opposed to your game built using your modifications). But I am not an expert on what they would consider wrong. Read the license information.
Cheers,
Ancient Lady
AGK Community Tester