I am around that weight. I am also 19 and 5'9-5'10. Underweight according to most people, but I am not reasonably capable of gaining any more body fat. I can sympathize with you, but...
It sounds like you're wanting advice that will a) make a healthy person gain weight using unconventional methods and a) prevent said person from working out, a healthy practice I would not recommend avoiding. I can't help myself, so I don't think I can help you.
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