I am currently trying to add muscle mass and have done some reading on this subject and most of the information I have found suggests that eating a low-carb or zero carb high protein snack at night is beneficial to preserving lean muscle mass and does not promote fat storage. The exception to this is if the amount of protein you eat is greater then your body needs to maintain your lean muscle mass then it may be converted to glucose and could then be stored as fat if it is not used to replenish stores in the liver or muscle. So if you are not doing resistance training you may want to be careful not to consume to much. Personally I drink a protein powder shake just before going to bed that contains 20grams of protein and 2 grams of carbs and have not experience much fat gain at night. Generally I believe if you keep your night time eating to low or no carb snacks you should be ok.