Who said that?
That doesn't sound quite right to me.
A "post" just generates gcode.
I highly doubt that the plasmacam control box just loads posted g-code over the parallel port then operates independently from the plasmacam software.
Especially not when it comes to dynamic height control.
As far as I know, the plasmacam control box is useless without plasmacam software.
So you'd have to
retrofit the electronics to use a different control system
Like mach3 (or dedicated CNC control station) to read the gcode and run the machine
Then you could use any CAM software - like bobcad or corel-draw+sheetcam
But making that work as good as plasmacam and incorporating dynamic height control would be 100 times more complicated than if you just got the plasmacam software to go with the plasmacam machine.
Notwithstaanding why the like new machine has no software - which I can probably guess but I'd rather not....