on CAM-Planes you can only assign nets. No need for drawing extra positive copper plane.
And on CAM-Plane you can only assign one net for the whole plane I think.
I recommend to use only split/mixed planes for your ground- and powerplanes.
They have a positive look and you can easy handle more then one planes at one layer if you want.
Then assign the net(s) you want on this plane.
Then draw the plane(s) as "plane area".
An easy way to draw a plane area on the whole split/mixed plane is:
- RMB - Select Board Outline
- select a piece of your board outline
- RMB - select shape
- RMB - create plane area