When you say you want the laser to engrave, it could mean two things.
In one case, you may simply want to set the power of the laser so it burns into the material. In this case the laser will operate at one power during the run. In another case, you may be talking about engraving the material
at different levels of power during the run. This essentially achieves different "grey scales" of the part while its exposed to the beam.
This is an example of a grey scale picture on wood, done with a laser:
http://www.quagmirepuzzleboxes.com/Plaques/Fam_L.jpg
This is an example of engraved piece of metal done without grey scales:
http://www.cascadelaser.com/Graphics/markeng1b.jpg
Doing things without grey scales means you have some knob on your power supply, you set the knob to a certain level, and let the CNC software control when your laser is turned on and off. You should be able to do that with Mach3.
Doing things with grey scales requires that the power is adjusted real time as the part is moved underneath the laser. This could be done with 1) a commercial laser engraver that comes with software to handle grey scaling; 2) writing the programming software yourself; or 3) possibly, by bastardizing the software that is used on the commercial lasers that handle grey scales.
I dont think 3 would be very easy.
owen