I don't think it is a good idea at all. 3D printers have different requirements compared to CNC. 3D printers demand very high speed and acceleration and CNC machines are not capable of that for several reasons, one is the far too heavy gantry and Z axis. Your machine will be ripped apart pretty fast if you start running it as a 3D printer, so I think it is better to buy a dedicated printer.
I had similar ideas but after some tests I decided not to continue with the plans of making my CNC into a CNC/3D printer combo, and after having used my printer for several months now, I think that was the right decision. 3D printing is a very time consuming process, even with fast printers, so already this is a good enough reason because to run the CNC for several days is not a good option. While both machines use the same principles of motion, they are not the same, so making a universal machine is in my opinion not a good idea. Doable, but not optimal.