You need to edit your post processor to do that.
I believe you want to change this line:
VAR Z_POSITION = [Z|C|Z|1.4|]
VAR Z_POSITION = [Z|C|Z|1.4|-1]
Here's a link to the manual, so you can make sure.
Imo, you'd be better off reversing the Z axis direction, so it works like 99.9% of the other Mach3 users.