First thing to do is use a cooking method without water. Second thing to do is to fill the ring FULL. If you're using lump, be sure to adjust the pieces for a tight fit. You should be able to cook a couple of butts without refueling.
Kevin has given you one method for adding charcoal. The best method for keeping ash off the food is not the easiest or safest (especially if you're using water) - The Squat. Basically, pull the top and middle sections of the WSM off of the charcoal bowl. Then you can tend the fire any way you want and not get ash on the meat. Personally, I wouldn't even try this with water and even without, the meat makes the WSM pretty heavy but it can be done.
I've never reloaded. I would feel very comfortable bringing a butt in the house and finishing in the oven. After a full ring of charcoal and probably 10+ hours in the WSM, the meat has all the smoke and flavor you're going to get. A couple hours in the oven is no big deal.
Good luck with your cook.