From what I remember the iphone cradle isn't compatable with the iPod as tomtom done one specifically for the iPod. Could be wrong but check tomtoms site
Anything is possible. Buy a Mac, get a developer license, start programming. I'm sure it is possible. On the other hand, for a reasonable price, with reasonable usefulness? No.
Buy the GNS 2000 GPS GLONASS Receiver. This is made especially for the iPod Touch and wireless-only iPad. By adding the TomTom app to your iPod Touch you have the best solution.