Here is the overview videos
There are several parts you will need to convert a CNC to a 3D printer.
This is a picture of my extruder. The bare aluminum is how I mount it to my cnc machine. The orange or copper piece hold my filament. As you can see there is a fan and a little stepper motor driving the filament into the extruder.
This is just some abs piping scred on to a little piece of aluminum that is mounted above the exteruder
4th Axis motor driver
Here is the motor driver needed for your fourth axis. On the software side you may need to make some changes to drive your 3D printer, or at least I had to consider that most 3D software will insert an E into the Gcode and mine wanted an A prefix. This hooks up like an other driver with steps and amperage settings.
12 volt power supply
This is a 12volt power supply for the motor, the fan, the extruder heating element and the temperature controller.
Here is my temperature controller. If it is permanently mounted it will need an on/off switch. It will have a screen with an adjustment knob to control the bed temperature. Mine was homemade so I have a plastic food saver as the body right now. If you want to build one check out THIS post.
My 3D printer bed is fairly large. I constructed it with aluminum plate. The plate is made stiff be attaching aluminum angle to the back.
The top of the bed is 1/4 tempered glass held on with double sided tape. So far I have not needed a heated bed. I put down blue painters tape and sand it with a 40 grit paper. This has held all my prints quit nicely. Some are even tough to get off but most are just right.
Bed Heater or Compartment Heater
Originally I was going to build a heated bed but after trying my bed above I realized it is not needed. However the other issue I faced is delicate parts curling because they cool down unevenly. This heat lamp was a cheap hack and way cheaper and easier to build than a large heated bed. It works great for small prints and at some point I might build an insulation blanket for the whole compartment as well as a temperature control for the lamp itself.