To add weight and still be healthy you have to gain muscles rather than fat. 

To do so the first thing you need is to burn out fat from your body via workouts that flexes muscle and burns the existing fat. And then you have to eat foods that are high in protein and natural steroid such as egg whites.. avoid anything that have cholesterol in it. And then do heavy workouts.

But to do above stated things if you try on your own do a extended extensive research on the matter. or choose the easy way and talk to a professional trainer.