I may be wrong, but I think buying a PPSH parts kit and modifying it accordingly would be the way to go here. It would be a lot easier to achieve your end result using the factory PPSH blow-back operation, than to convert an SKS style gas operating system to look similar.
GunBroker.com guns: Polish PPSH-41 SMG Parts Kit (item: 95932276 ends: Apr-03-08 01:27:33 PM)
Not that an SKS conversion couldnt be done, but It could be done more economically fitting and less labor intensive to use an existing kit.
And to answer the question whether or not the SKS can be converted to other calibers, the answer is yes. The response in question form is, how much work do you want to do?
If you are willing to fabricate parts, and experiment a bit, it can all be done.
Hope I was some help.