Not sure if it helps anyone, but I've been looking at this project myself and thought I'd post up a few of my findings...
If you have a pre-facelift car, this retrofit might be feasible. Reason being, there is an ACC retrofit kit available (although it may be difficult to get hold of). The kit comprises the radar head, mount, adjusting frame, a wiring harness to replace the one already in the front bumper (which supplies the parking sensors, horn, fog lights and the new radar head), a new wiring harness to link the radar head to the bus, and some trim pieces for the lower grille.
If you have a post-facelift car like i do however, things get complicated. A new system was brought in, ACC 2, which is more sensitive and uses a new radar head with four antennae (as opposed to three in the old ACC). This is good, because it makes the system more accurate when it comes to reading whats in front of the vehicle. Unfortunately however, from what I can gather it uses sensors in the brakes which may or may not already be fitted to the car (I couldn't bottom this one out - either it picks up on signals from sensors already present in the DSC system, or extra sensors are fitted to cars which have ACC 2 specified at the factory). Oh, and there is no retrofit kit part number available for post-facelift cars that I can find.
I found a cheap ACC 2 radar head on ebay.de so took a gamble. My reasoning was that everything else can be purchased from dealerships, apart from the wiring harness which links the radar to the bus. So I looked in to making my own harness. trouble is, on pre-facelift cars there is a splice point for the two bus wires behind the headlight but I can find no trace of this splice point on the wiring diagrams for the post-facelift car. Even if I got round this, there is still the brake sensor issue, then of course the car would have to coded, and after that the radar head needs calibrating (haven't got a clue what this entails but I've heard mention of hours on some kind of calibration rig and ££££££).
Anyway i did all my research earlier in the year then had to break off to work at the London Olympics. I might come back to it in the new year but until that I've shelved it. Hopefully someone else might pick up this gauntlet, it certainly would be a cool retrofit.