there is approximately 3000 calories in 1lb of adipose tissue. Since digestion and whatnot remove some of those calories, that just means to gain one pound you'll need to eat 3000 cal MORE than what you burn. The average male requires 2500 cal a day (more if you're an athlete or work a labor-intensive job, less if you are sedentary and don't exercise). So to gain one pound a day over a week, you'd need to eat on average 5500 cal/day = 38500 cal/week.
both figures border on the caloric REQUIREMENTS of special forces and Ranger units, and they still get far less than that while in training and on patrol.