There used to be a kit with a different pedal assy that had a switch and wiring harness built in, to do it correctly. Sorry dont have a part number but a Hyster dealer with a good parts guy should be able to get one for you.
Don't think it matters, is it a monotrol? If it is, there are 3 pressure hoses on the pedal, you might be able to tie into. One of them is reverse pressure port, you might be able to use a brake line to extend and install a switch at the trany end.
Putting a pressure switch on the reverse port will only activate the switch when there's pressure present but not when the inching circuit is activated. Don't back up alarms need to be wired so that the beeper sounds when reverse is selected, not only when the truck is moving?