Having already created a multi voice performance, how do I switch on/off the SUSTAIN PEDAL function on a voice without affecting the rest of the voices.
There are no "voices" in the Montage. You are referring to Parts. You can activate or deactivate controllers on a per Part basis.
Press the PART SELECT button [1/1]-[8/8] that corresponds to the one you want to edit 1-8
In the lower left corner of the screen select "Common" (blue)
Select "Mod/Control" > "Receive Switch"
Find Sustain and deactivate it
If you continue to call them "Voices" (sic) you will never be able to refer to the manual or the front panel and learn your Montage. You're editing a "Part".