S owner here...
...the answer to your question is that although you can probably find someone to do this, i wouldn't recommend it.
First, the head unit doesnlt have a F/R fader, so you'd have to put one in. Then, you'd have to run the wiring from the head unit (after removal!), to the rear doors.
Then, you'd have to cut those lovely door panels.
And, the results wouldn't be so good, b/c the head unit has low power, the sound coming out of 6 (remember there are mini tweets near the inside door handle) would be about half of what you have now..it takes more juice to move 6 than 4.
So, you'd have to add an amp...
All for a bit of rear fill?
The sound on the std. unit isn't bad..and there's the neat 'Occupant' feature, which helps with balance...scroll thru the menu and adjust it, along with the bass, and treble.