I know that having supply trucks in a Formation will keep any units in that formation supplies, as well as any units subordinate to it. Typically this has resulted in my filling to the brim the top level unit with supply trucks. However I am not sure how to handle 're'supply after that. If my supply gets used up and I have to replace it, how can I easily move individual supply trucks into that formation?
What I was wondering is if I were to 'attach' a unit of nothing but supply trucks to that top level unit, would those trucks be drawn upon to supply the troops below?