well it is only easy if when they rebuild it they dont make the buttons do things.
I am guessing you will want the volume to be the volume for the stereo all the time and just have the PC set to match the normal radio levels you get so you dont get blasted.
If pressing the buttons wont do anything when it is in Aux mode then you can fit a second set of push switches to the back of the buttons that can be interfaced with a hacked number pad to use at hotkeys for things like "bring up gps, music etc or play. pause etc.
as for the other knob...you could drill the centre out of a gear and put it round the shaft or something inside. then use another gear up to a rotory encoder off a mouse or soemthing. That way you could use the tuing knob for next track etc etc. You will of course need to switch all this so when using the radio it doesnt affect the carPC.
that is easy using a kvm that is switched on/off when you press the aux in button.
lot of tinkering but it could be done easily enough.