I have seen rear trunion blocks to convert them before, but they stick out from the end of the rifle and look kinda strange. Other than that the only way I know of to do a swap would be a new reciever. Probably your best bet, since they only go for $60-$80 bucks at the gun shows.
On the Underfolder deal, YUP< they do sell that Funky aftermarket rig, But it's not a real good folder, as the Mag gets in the way when you fold and it does stick out too far! The only real way to do it, is if yours was a Preban that was shipped in as a Post Ban and will have the Knock out area that you need to have the Holes to Put the Push Button for the folder into! You could Modify a reciever ro have these, but even converting the Post Ban back to Pre-ban design is not legal unless you comply with all the ATF requirements! To Be honest, from someone who has tried and carried Both the Fixed stock as well as the Underfolder, the work Just isn;'t worth it in my Opinion! The Underfolders always rattle, are Flimsey against your Cheek when you fire then (Especially in FULL auto) and Thier are quality, aftermarket ones from Bulgaria and other places that Fold Nice, fight right in where your fixed stock did and are comfortable, Check out K-Var or Tapco for a look at them! They will also advise you what you need to do to make it a legal conversion, no sense spoiling the fun with legal issues, right?