The CNC machine will, of course, need some hardware upgrades such as printhead as well to fully turn your CNC into a 3d printer, but this software upgrade will make your CNC hacking much easier.
Software add-on and instructions can be found here:
Here is a much more detailed Instructables guide page made by Nuri:
Here is a video by Donald Fournier showing how he uses a Hoss G0704 as a 3d printer:
Here is a video by Chris DePrisco that shows, in great detail, how he attached an extruder to his CNC mill. He also shows the test 3Dbency and other 3d printed upgrades.
Here is another way to use Mach3 on CNC machine as 3d printer, and description of hardware needed. One could combine those two methods.
or have a look at the extensive video showing a modification of CNC table plasma router to a 3d printer:
Jon also converted his CNC into a 3d printer and uses Mach 3:
|Mach3 UI was always fascinating to me ...|