Yes, it is possible depending on the manufacturer of both airsoft and real steel. However, be careful as many lower end airsoft use cheap plastic and NO heat shield. IMHO I would spend the few bucks and get a true set to be safe.
You would be surprised. Mainly due to the fact of price points. I know it is especially popular in the Walther MP5-22 becuase at first release there wasn't a lot out for it, and it was expensive. Lots of people would use Tokoyo Mauri or similar higher end airsoft brands (and yes cheap knock offs) and modify them to fit the rifle. Handguards, side folding stocks were very popular as well as the sliding and full stock becuase all 3 could be interchanged with just two pins. And mostly the claw mount for optics. I saw real ones going for almost $100 where as airsoft versions of this mount could be easily bought for 15-30$