I am curious how they modify these to fire from a closed bolt. Do they add a hammer or something?
All of the "open bolt machineguns" which are converted to semiauto have the addition of a striker - a separate chunk of metal that hangs back on the sear while the rest of the bolt goes in to battery. (DSHK, RPD, PKM, Vz 37).
On some rifles this striker is made of a piece of the original block, bandsaw cut off of the original bolt group - just whacked right off the back (RPD, DSHK). On other rifles, the striker is a newly manufactured component.
Essentially it turns the back of the bolt, which generally operates in open-bolt slam-fire mode, to a two piece affair that has two springs - one for the bolt assembly, one for the striker.
Other closed bolt / hammer fire / sear fired guns had trigger pack modifications to fire in single-shot, but the bolt itself received minimal (if any) modifications to accommodate single fire. (My german MG-34, MG-42 are like this). This moves the complexity of the changes from the actual operating mechanism to the trigger group. It also means that replacement parts for the wear pieces are MUCH easier to obtain, and don't require custom fitting (generally).
(The trigger group on open bolt machineguns such as RPD, DSHK, Sten, vz 37, etc is really nothing more than a lever that moves another lever out of the way from a ledge on the bottom of the bolt... people don't understand how insanely simple some machinegun designs are, nothing "complex" at all about them!)
Closed bolt machineguns are much more complex (if you ever get the opportunity to examine an MG34 bolt, you'll see more machining required than in an entire M14 rifle...). Whereas open bolt machineguns are exceptionally simple.
Funny, but the simpler the machinegun, sometimes the harder it is to make it work semi-auto.