I agree with Marc0198 and Godfrey. Gaining fat ain't gonna work that well, trying to gain muscle with exercise and working out is probably the best bet. Also, DO NOT rely on the junk food that corporations like to label as nutritonal supplements like Scandishake and ensure. They are full of inflammatory crap and worsen digestion and can contribute to lung exacerbations. Eating high-calorie whole foods is the only good option.
Also calories ingested DO NOT EQUAL calories digested. We may intake 3000 calories, but if 2000 of that is from processed junk food (sugars, refined carbs, vegetable oils, processed protein powders, processed dairy, etc.) then you may only be absorbing a fraction of what you put in your mouth, and at the same time causing yourself serious digestive problems and systemic inflammation. I discuss this at this post: http://cfnaturalhealth.weebly.com/blog/calories-ingested-vs-calories-digested
Also calories ingested DO NOT EQUAL calories digested. We may intake 3000 calories, but if 2000 of that is from processed junk food (sugars, refined carbs, vegetable oils, processed protein powders, processed dairy, etc.) then you may only be absorbing a fraction of what you put in your mouth, and at the same time causing yourself serious digestive problems and systemic inflammation. I discuss this at this post: http://cfnaturalhealth.weebly.com/blog/calories-ingested-vs-calories-digested