Have you considered putting your source code under version control? If you use a service like BitBucket, then you have all the benefits of version control, and you can also easily update your Xcode project to the latest version simply by pulling it down from the server.
On my current project, I have moved the source directory so that it is actually shared by all the different platforms. That way I avoid having to duplicate files altogether.
There is no one off the peg solution for this. I do it using the combination of a shared source directory and some python scripts that generate the Android makefiles etc. I don't have any experience with AppGameKit on Mac OS X though, so I couldn't advise on the specifics of that I'm afraid.