I'm looking for a way to add focus/record capability to my Ronin-M. Can a USB cable and control knob provide the same focus control as found in the Ronin-S?

I'm not an expert in this by any means, but near as I can tell, you're probably going to need to fork out for a Nucleus or DJI Focus and maybe even a Remote Start/Stop. The thing about the Ronin-S is that DJI have grossly undercut the professional filming market with the low-priced accessories. You're going to get more customization/expand ability but for a cool buck.