I would have thought OEM 'parktronic' sysytem is not possible to retrofit. And if it is possible will cost a small fortune.
Better to get a decent after-market system; I'd recommend Cobra/Vodafone, they do two systems. Get the better system and it looks OEM all day long. (Better looking than Mercedes OEM too). Easy enough for DIY if you want (I've done three installations without issues).