Well my guess is you are talking about a Mak 90? You are allowed to change/add these things as long as you still follow the 922r compliance. You have to change a certain amount of parts, in order to be left with only a certain number of imported parts to make it all legal. Here is a website I have used...
http://milsurpstuff.com/922r%20Compliance%20Info.htm
http://milsurpstuff.com/922r%20Compliance%20Info.htm