You should be able to do this quite simply one of two ways, the first with a 3 posn. Mom. L/R and center off switch such as Telemecanique/Square D etc, and just stack contacts as needed.
It also would be fairly simple to do with a 3 position single contact switch if you intercept the wiring at 219 221, and have these conductors controlled by two small multi-contact relays, these would be fed from the latter switch and they in turn would feed the Hi/lo contactors.
IMF IMR, strangely they are marked as one would For.Rev rather than two speed?
The interlocking would be done by the small relays instead of the single multi-contact switch.
Looking at the diagram again, the switch appears to be L/R maintained, not momentary?
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.