Adding Rockford Fosgate package to non-rockford MMCS
 

I've just got a 2010 Citroen C-Crosser, which is basically a Mitsubishi Outlander, and it came without the Rockford Fosgate amp and sub, but with MMCS.

Can the amp and sub be purchased from anywhere, does it need any significant change to mmcs or does it just plug in, and has anyone else done this upgrade?

Yes it can be done. It may require having the dealer program the "premium sound" option into the ECU, but I believe its not required. Check behind the MMCS to see if all the ports in the back are being used. If they are, there is a connector that goes from there directly to the AMP and sends the audio to there as well. The amp connector is probably under the carpet of your front-right seat, so make sure there is a connection from there to the back of MMCS. If there is, it should pretty much be plug and play.

Rockford Fosgate complete factory sub for sale
 

I have a mint Rockford Fosgate sub that came out of my 09 Outlander. I took it out before it went to the scrap yard when it was totaled. The 09 had 7k miles on it. I was going to put it in the next one was going to buy, but bought a Hilander instead. I would like to sell it, make offer thanks.

Should be easy enough but it depends if you have the wiring run.
Look under the seat, should be 2 plugs (one 18-pin and one 14-pin)
If you don't have the amp plugs it looks a bit trickier but there are a couple of us in the uk that are currently working around it..

also, its not just the amp and sub, you need all the door speakers and tweeters (or some 2ohm units)

Info about rockford installation
 

Is there available harness for a non rockford audio for outlander 2009 model