I have no idea about the AK, but the M15 (civilian designation) takes a few parts to convert it to select fire. Some civilian parts must be replaced, like the bolt carrier, hammer, trigger, sear and safety lever, and a few must be added.

While this may be possible, having any machine-gun-specific parts in your possession is the same as owning the machine gun itself. That's legal under some circumstances, but if you don't jump through all the hoops, it's something like 10 years in jail.