I don't know. I used to do it by providing a complete path for the routing bit, using a track of a designated thickness on a designated layer. The PCB manufacturer would use that layer to set up the routing bit path. I think you would have to work with your PCB manufacturer to explain what you want, so they could program the CNC machine to do it properly. You might want to place a standard hole (with no copper around it) to act as a starting point, so the routing bit doesn't have to drill its own starting hole. Also there may be restrictions on routing bit diameter, because the routing bit has to be thick enough that it doesn't break as the router drags it around the board, cutting its channel.
These are just my speculations. You're best to talk directly to the people who will be manufacturing the boards.