How can I override the cut/fill slopes of the ground type without changing them in the Ground type Editor?
You can alter this by changing the Slopes left/right % Final from Auto to something else in the Template Editor. Alternately you can change fill material (and therefore slope) by using Edit|Asign Fill Types or you can change the GND field in the associated traverse. (You must re-read your Location if you change the P-Line Traverse).