Extending an existing CNC controlled by the parallel port with an arduino

I want to add a 4th axis to an existing CNC that is controlled over the parallel port. As anyone ever try to use an arduino for that (running grbl)? How would it work for the gcode to be sent on the parallel port and on the usb port at the same time? Is this possible?

Thanks for your help!

P.S. I want to go this route because I can't modify the existing CNC, it's not mine... I would just like to build and use a 4th axis when needed using a different profile.

Do you just want to index or do you want full control?

You can't use the USB port for real time control and grbl does the same job as Linuxcnc so its not going to help you but I might have an Arduino script that would allow you to index. Preferably the command for the Arduino to index would be controlled by a spare pin or a relay on the CNC controller (some commercial machiens have relays that can be triggered with an M code, but you can also trigger it by passing a tool past a proximity sensor when you want to index.

just use a cheep china 5axis boaed about 5USD and you got lots of options up to a VFD control
12 free programable outs and 5 in