You will face a couple of issues:
1. Generally plasma needs to be light and fast v's heavy and slower like a router.
2. Your older machine probably does not have a CNC port but there are ways around that
3. Your older plasma cutter probably uses high frequency start which is hard to manage with CNC due to the electrical noise it generates. Pilot arc or blow back torches are better.
Normally you would want a torch height control module. This will require a voltage divider to be fitted. If you are only cutting thicker plate, you may get away without it.
Ata simplistic level, you could probably just mount your torch and add a relay to turn on the torch under CNC control
Normally Plasma's return an Arc OK signal to the CNC controller to tell the machine it has a good arc and can start cutting but as a simple solution, its not necessary.
So at the most basic level, attach the torch to your Z Axis, touch off in the normal way than run some gcode to raise to pierce height, wait for pierce delay and then drop to cutting height and start cutting.
PLasma is however much more complex than this to build a robust machine.