I've made 3 CNCs in the past, the last one I bought a mk3 plastruder in hopes to make it a 3d printer as well.  Though that was 2 years ago and am sure things/software has changes since.  At the time I had a hard time getting the open source 3d printing software and the professional Mach3 program to talk the same language.  You'll have to know some basics of the CNC electronics to be able to mate the 2 together; know which free wire from your CNC control board to connect to which wire on the extruder head for it to communicate, etc.  Also a 3d printer is often faster (though not always the case) in movement than most CNC machines.  If you have a home-brew and using slow screw parts for your movements, it may be way to slow to be of any practical 3d printer. I just got a Makerbot Replicator, a fist-sized print could take 4 hours easily to print, and that's a god 3x faster than my 1st couple CNC machines. 

Fast forward now with a little Makerbot experience and past CNC building knowledge; if you can afford to get separate machines.  Though both machines are built nearly the same style, they're not the same type of machines.  A 3d printer is faster but doesn't need any force needed to print so can use weaker motors and mechanics.  A CNC needs strong motor for it to cut through the materials, which often times translate to slower moving parts.  Unless you're willing to be patient, and willing to tinker with how to make the 2 hardware speak to a common software...

In short if you're still interested in modifying a CNC setup; all you'll need is the extruder print head.  The rest you'll have to figure out the software end as mentioned above, and some electronics wiring to hook up the print head correctly to your control board.  The current software is alot more friendlier than the ones I tried out 2 years ago, using them to slice up the model shouldn't be as extremely difficult but still could be tricky depending on your skillset.

Forgot about a few other things.  You'll need to control the extruder head temperature and optionally (recommended for better quality prints) a heated surface to print on. 

To use it to make plastic extruding molds so you can use more than PLA and ABS plastics...