What is "this" ?
An Arduino is just a microcontroller. It can talk to your computer on one end, and can twiddle bits on the other end. If you use all the pins on it, you can get something like 9 axes with step/direction control out of it -- assuming you have the right controllers on the other end of those pins.
I think what you're trying to do is build a 3D printer from your existing 3-axis control, by adding a plastic filament extruder driven by a stepper motor? This would add a 4-th axis to your control system?
Does your HobbyCNC controller support 4 axes? If so, you may want to try just hooking the stepper controller up to the stepper motor and the HobbyCNC.
If it does not, then you need to find another way to control that fourth axis. An Arduino, with the right computer code, can certainly do it. The problem then becomes: What software are you using to run it all? Can that software be told to talk to HobbyCNC for XYZ, and your other controller for A (or whatever you call your fourth axis)?