We're not just talking .air files here.
Premium planes don't have config files for aircraft, engines, flight_model, or systems.
So, something as simple as changing your tail number or the crew (or, like in the case above, adding an autopilot) is impossible.
(Actually, the unencrypted config files are available on some darker corners of the web, but you shouldn't have to jump through those kinds of hoops, just to do some tweaks to the planes you paid for.)