Casein protein?

ive been taking whey protein immediately after workouts and as a healthy snack for a while..ive lost alot of wewight and i need to gain around 45-50lbs to get back to my healthy weight...i want to feed my muscles at night and i was wondering if buying a tub of casein protein would help....i usually mix my protein shakes with skim milk...is it a good idea to take a casein shake about 20 mins before bed with skim milk? i really need to but on muscle m but i dont wont to take any weight gainers .