I don't see why one can't gain weight in much the same way as one loses weight. Just add calories instead of subtracting. Your appetite should adapt. The only difference is that you need to add more calories than you would subtract for weight loss because the body isn't as efficient at storing energy as it is using it.  I understand that you're making a statement on how fats eat, but it's actually easier to lose weight than gain it, which is just as bad.