There are folks doing that type of conversion(check on
www.hk91.com). I don't think a G36 trigger group works though...unless you wanna go Class III.
There are other G36 parts that work...the foreend/handguards, the optical sight/carrying handle etc. Usually your US parts count would come from the trigger/hammer/sear plus others(gas piston etc). Special Weapons is making a kit but I think that includes their AR-15 mag conversion too.