you have ad-shaped hole in the board ?
all you have to do is create the features from the board.
Design - Board Shape - Create Primitives from Board shape,
then select Include cutouts and send it to a mechanical , or better to the keep out layer.
the board house will read the GKO file and use that as contour. don't worry about size of the routing bit. the cam processor is smart enough to mill outside or inside depending on nesting.