Not really.
I cannot understand why they want to make the tool lengths 1" shorter, & make Mastercam go 1" further ( NC code may not be quickly read, you need to be switched on at all times when setting up )
- why not do it normal like everyone else, set correct lengths, run Mastercam as usual......no stuffing around with modifying posts, NC code can be read as correct depths, no calculations, everything falls into place anything Z- is in the material area ( & possibly cutting ), anything Z+ is clear of the part

The shop should have a worked-out procedure in place for setting tools, there is a great chance that a crash will happen, especially if operators do things differently to each other.

Mastercam does not know what your tool lengths are. BUT it does know the relationship of the tool tip to the part origin
A couple of options for you to consider
- depending on the control, add / measure 1" to the tool offset. so the machine is using the correct numbers (best), Then raise the part origin by 1" to prove-off the NC code
- lower the part origin by 1", then measure your tools( don't add the 1"), then raise the origin back to original setting ( this still makes your tools 1" shorter than actual ), but procedure is still feels wrong. Too many steps & can still go crunch

but to answer your original query
- (in Mastercam) place your part 1 " below the origin, so the origin IS the top if the 1" gauge block, program as normal ie facing the part would be at Z-1.0