Using A axis as second spindle with stepper motor

Hi,
I'm using linuxcnc since a while. I upgraded a few weeks ago: Now I'm using a Rpi and a MESA 7C81. I'm having 3 axis and a fixed vertical spindle motor and a separated a-axis. All 4 axes are running with stepper motors.

I'm using inventor-hsm for creating G-code which works pretty good. Unfortunately, it is very limited, when using it in connection with the A-axis. Although the software is able to unwrap a geometry I can only run it to mill out closed pocked (also when it is wrapped). Very frequently I need to reduce a diameter and there is no operation available for this. I can only do this in indexed mode (several phi-angles and slicing in z). 

Luckily the software can also produce code for turning machines, so I could create code for such a machine. The only thing I need to do is to turn on a horizontal spindle, you see my idea?

So my question: 
Is there a possibility to configure linuxcnc in such a way that I can use my A-stepper axis for running as horizontal spindle?

What I need is a G-command turning the horizontal spindle on in a certain speed and a G-command for turning it of, during the turn-on the stepper needs to run continuously. I know its a bit absurd

Georg