The PPK is a blow-back design. It is a nasty gun to shoot in .380, and though there are 9mm blow-backs, I really don't think a PPK with stronger springs, etc., to accommodate the 9mm would be pleasant. Re-engineering to convert it to a Browning-based locked breech design with lugs, cam/link, guide rod, etc., probably isn't practical due to the small size of the gun. JMHO, I'm not familiar with the Rohrbaugh design, but have you looked at a Kahr PM-9?
If Walther/S&W wanted to do a small 9mm, they'd be better off starting with a clean sheet of paper, IMO.