It's not possible to transfer iTunes/App Store purchases to another Apple ID account. I think in part it has to do with the terms of the deals Apple has with the record labels and app developers. It's also likely a safeguard against the hijacking of the contents of an account.
I'm not sure what you think you'd gain, other than knowing all your Apple stuff is on an Apple email address, while all your Google stuff is on the Google address. Further, if you use iCloud and also want your iCloud stuff to be on the new Apple account, then you'll need to copy all your contacts, calendars, documents, photos, etc. to the new account... really doesn't seem worth the bother.