This is the SolidWorks Assembly of the Prusa i3 MK3S MMU2S. This one didn't need any of the individual files converted to STEP, as the MMU2S specific parts are already in STEP format on the Prusa GitHub.
The MMU2S unit is exactly the same as the MMU2, so no need for any changes there. The extruder is the only assembly that changed.
As always the Prusa i3 design is Open Source, so the Assembly is too. I would appreciate it if you list it as a source when you do any derivative work so that others can see the cool things that are done with the assembly!
UPDATE 4/10/19: Thanks to user kvn5x4 for pointing out that the extruder assembly was slightly crooked. I added a parallel mate to fix it and have re-uploaded the assemblies.