RepRapFirmware (like most firmwares) uses the switches for homing only. While it would be possible to wire 3 additional switches to a Duet, you would need to modify the firmware if you wanted the switches to be active all the time. IMO it isn't necessary, because once you have homed the printer, the firmware will prevent it from moving outside limits.
However, I intend to add a facility to attach a pause button in the near future, and possibly also an emergency stop button. You could wire 3 extra switches in series and connect them to the emergency stop input.
But my advice is not to bother with extra switches, just set up the config file correctly, and use rubber pads or similar as mechanical endstops.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
