Nope,
Afraid not, as you're violating a Federal law by doing so. 922r statute states that the rifle cannot be "significantly" modified without reducing the number of foreign-made parts on the rifle to "10 or less", thussly making it an "American made" rifle. This is how those WASR-10 Ak's are making into the country (many US parts on them such as the trigger group, gas tube, pistol grip etc.) What is "significant" you ask? The law doesn't say (anywhere I could find anyways), but the part groupings are clearly laid out by the BATF. Check this link for details:
www.victorinc.com/SKS-FAQ.html
It explains the whole foreign-made rifle mods thing in graphic detail.
Also, as I understand it, even if you play the "10 or less" game successfully, you still can't put a folding/collapsible stock on the gun as NOW you've violated the Mass AWB by "making" an assault rifle! IANAL, but the explanation I recieved was that since the gun retains C+R status, and was not an AW (no pistol grip), changing it to a pistol grip style weapon and adding a folder style stock "makes" an AW. I asked the 3 or 4 people I spoke to about the fact that I did not intend to put a detachable mag on the gun, so would that qualify it as NOT an AW? (One of the requirements of the AWB) Nevertheless, I was told that it
still would violate the Mass AWB, entirely seperate of the Federal 922r violation(s).
As I understand it, the AWB requires the gun in question to have a pistol grip, a detachable magazine AND one or more "evil feature" to be "banned". Since all AR/AK/CETME-style guns all have pistol grips, it was "one more stike and you're out" with relation to the other evil features (Bayonent lug, GL, folding/collapsing stock et al). I thought that the detachable mag was a given, otherwise ALL of these weapons would be under the ban, or we'd end up with those bizarre "break-open" fixed mag AR's they have in Commie-fornia.
I have a really sweet Tapco stock for my own SKS, sitting in my attic as a result. If any of the resident Legal Eagles know about this, I'd be greatly appreciative. I'm not sure that I'd bother to play the "10 or less" game and buy a whole bunch of U.S. made parts for the gun, just to put a new stock on it, but the original stock is really short, and I'd much rather have the Tapco on it if I could.