Yes it can be retro-fitted, just look up parts on realoem.com or get the dealer to quote you for the m bumper skin and fixings, I did this on a previous E87. From memory I needed an adapter for the under tray and a couple of air ducts. It took me a couple of hours to change over. There's also a few F20 m sport bumper skins appearing on eBay.
120D Sport (F20) Bk Sapphire/Coral Red/Piano trim/Sport Seats/18” 385 wheels/ Sport 8 speed step auto/Park Assist/Internet/Office/3D Nav/DAB radio/ Harmon Kardon/M Sport suspension/Anthracite headlining/ Driver comfort package/Interior comfort package/ Visibility package/Sun protection package/BMW Pro Media package/Variable m sport steering/PDC