There was some discussion about this in this thread if you’d like to pick up where we left off. These instructions are to create a custom sustain pedal from scratch.
To get a standard sustain pedal to work I think you’d still have to wire up a custom cable adapter. Where one side is TRS and the tip and ring are shorted and then on the other side is TS. It would have to be used with a “normally closed” sustain pedal. That way the footswitch is off when released and on when pressed.
I might have a Yamaha FC5 normally closed sustain pedal around here. I’ll try to find it and report back.