Here's how it works. An SKS with a folding stock in not legal for importation. ATF has decided it is not a "sporting firearm" or in the case of the Yugo, not a C&R firearm. The same applies to an SKS with detachable magazines.
USC 922(r) says that you cannot assemble from parts a firearm that is not legal for importation. That means that even though your SKS is now in the country, you cannot put a folding stock on it.
ATF then came up with a way to change your firearm from being "imported" to "Made in the USA".
To do this, ATF has made a list of parts that they consider major components of a firearm. You take the list and determine how many of the listed parts your rifle contains. It might be 13 or 16 or 19. Whatever.
ATF then says that if your rifle doesn't contain 10 or more "imported" parts, it is not considered an imported firearm. If you figured out your rifle had say 16 parts on the list, you replace any 7 parts on the rifle with US manufactured parts and your rifle is now "Made in America". Be sure to keep the receipts and/or other documentation showing you have replaced the parts.
You can now make any modifications to your rifle you desire that are legal for any other US made rifle.
The parts list is:
(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunnions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates