conversions/ starting from scratch

Well for the ppsh it would appear that agm will be producing one in the near future.

Breath holding......COMMENCE

I'm not convinced as to how useful or skirmishable a recoilless rifle would be...given it's size and the lack of useful ammunition you'd just be lugging around a big pipe with a few bits on it.

A version using MOSCART would certainly be possible. I am not sure it would be practical for the reasons given above. As the 57mm recoiless rifle saw relatively little use in WW2 all the engineering work needed may be better directed to infantry small arms.