What do you have for a plasma? Is your CNC a mill or a router?
If it's a mill, you may not be able to hit the speeds you'll need for plasma. If you do set it up for plasma, you'll only do it once. Plasma dust, which are tiny metal particles, will get everywhere and you'll spend the next day cleaning the mill.
If it's a router, it's a little easier convert and clean and your feed speeds should be faster. Even with a router, it's not something you'll want to do all the time. Having done that before, I wouldn't even run a router table in the same vicinity as a plasma table. Unless you have a good downdraft or water table, which you probably won't just doing a conversion to a mill, the plasma dust will be almost unmanageable.
Converting the plasma to CNC depends on the plasma. Any of the reputable manufacturers sell a divider/CNC interface board. Depending on the plasma, that can be anywhere from $100 to $300....or if you look at a service manual for the plasma you can figure out how to do it without buying their board. We used to build our own interface for Thermal Dynamics and Hypertherm, but started using their factory boards about a year ago.
Sorry....Just noticed you said your plasma was a Miller Spectrum. We don't mess with miller. You'll find that even the techs at miller are largely clueless on setting up their machines for CNC. They did tell me once that ARC OK, Start/Stop and arc voltage were all available by tapping into the main board. You would have to make your own divider board. Miller does have a CNC interface "Box" that either plugs into the plasma or comes with their machine units, but when I asked for availability and pricing on it, they didn't know what I was talking about.