Offsetting Z0 point

I am trying to make a Lithophane. I am using PhotoVcarve to generate the toolpath and g-code. They requires to start the machining 2mm below surface with maximum range (low to high peaks distance) of 2.5mm. Starting with 6mm thick material, the thinnest point would be 1.5mm (6mm-2.5mm-2.0mm=1.5mm). I have to assume that Z0 point is set based on the smart clamp. How can I take my 2.5mm thick part and recess is 2mm below surface? in other words, How can I reset Z0 at Z (-2.0mm) point?
This link to an image might clarify what I am trying to achieve: https://www.dropbox.com/s/gt48ufkylh7259p/Machining_LithophanesB.bmp?dl=0

I watched the video and also followed their written instructions. Following the video, will not yield a correct Lithophane with Easel.
However, most CNC machines software interface have the capability to make multiple tool paths, usually one for making the material planar, roughing tool path and finishing toolpath. The Z0 axis could be re-zero after shaving the top layer.
Easel recently added the second toolpath in their interface, but not when importing G-code. You could obviously run the program multiple times with new G-code every time, However I do not see an easy way to tell the software that we already removed 2mm in the previous G-code import.
I am trying to avoid the roughing path from “machining air” from the first 2mm already removed in the previous toolpath and for the finishing path machining the volume already removed by the previous two tool paths.
Any suggestions?

Thank you very much, I will try it.
I will save it as two different paths. This way I will have the opportunity to change the tool.