If you decide to go natural, try increasing your caloric intake by 500-1000 calories per day. This will lead to a gain in bodyweight of 1-2lbs. per week. If you take a gradual approach like this, you gain more lean muscle mass with less bodyfat. Try adding meal replacement drinks to your meals to increase the calories for the day. Once you've achieved your desired weight, you can maintain at that point. Exercises such as squats and deadlifts involve a large amount of muscle mass and will stimulate your appetite so you can eat more calories.