you can fit an aftermarket changer in there yes, but would need some addition cables/adaptors. I believe Autoleads do these which allow you to run a Sony? changer. You would need to run the cables to the boot yourself though.
The factory VW changer in the DIN slot above/below your existing cassette radio would be a simpler and easier way of doing it though.
Not sure if both or only 1 of the versions will run the changer though. I think there was Gamma and Beta versions.