Can a model be used as Stock if I want to modify an existing part?

I have a number of existing parts that need a new cavity cut into them. I’m having trouble figuring out how to do this. Seems like having the existing part be the stock would work well but I don’t know how to do this.
In conjunction with this I want to hold the part in a fixture. A portion of the fixture will be above and offset from the place the cavity needs milled. Is there a way to designate “No go” areas for the end mill so that I don’t crash into the fixture?
Any help would be greatly appreciated.

Hi, Dale. I recommend using a pocket or trace operation to create an operation just in the area you want. Then it’s just a matter of choosing your origin for the gcode. As of right now, there is not a “no go” area feature.

Thanks Stewart. Pocket looks like it’ll do what I need. I’ll just be careful with the origin to avoid the fixturing.