First of all, it wouldn't happen that fast, so slow down. Second, don't over-eat until you puke. That's not healthy, and could lead to bulimia. Third, talk to your doctor about why you want to gain weight, and they can refer you to a dietician to help. This is the most sensible way to ensure things go smoothly, so don't go stuffing your face with junk food until you "explode". Finally, be sure not to go overboard. Being overweight can lead to diabetes, heart complications, breathing problems, and a myriad other things.

I would speak to a dietitian on how to best build weight. Don't want your digestion system to suffer, good luck

If it’s really that impossible for you to gain weight you should see a doctor. You could have an overactive thyroid.

Easiest way to do it without feeling like you are going to explode is to drink a slim fast or complete meal smoothie after you have your meal. That way you double your calorie count.

Protein powder in in full fat chocolate milk. Calories galore bud.