I have to add fuel for a brisket,and I Don't use water. So I am sure you will have to add. One big advantage to not using water is you can lifter the center section off without worrying about spilling hot greasy water. of course you can still probably be alright if you time it when the water is very low. I have garage door handles on mine for lifting.
And all that said,there is nothing wrong with just adding through the door. It just takes a little more effort.
"How much" fuel depends on a lot of factors. Type of charcoal,wind,outside temps,size of brisket. And then there is the option of putting it in the oven if you wrap it (I do). That would be a good time to "refuel" if you go that way. I and I believe most folks on here put them back on the smoker for a little while after wrapping. But in reality there is nothing that says you have to do it that way. You could unwrap it and finish it in the oven. They say most all of the smoke is absorbed in the first few hours.
Not sure why,but I hardly ever cook a brisket in the summer. I usually cook them when it's really cold crappy weather. Which obviously takes more fuel.
I have yet to try the "high heat" method.
https://www.virtualweberbullet.com/brisket-high-heat/
But you could probably do that without refueling and finishing in the oven. (but you don't put water in the pan) I have yet to try any of Chris's recipes that didn't turn out great. I will probably do the high heat method very soon. And the advice about the foil pan sounds like the way to go to me. 2 and 1/2 hours on the smoker,at 350 and cook the rest of the way in the oven.
Sorry if I am babbling