If you're already feeling full on your current solid food intake, maybe you could try drinking your extra calories. Drinking calories is usually much easier than eating since your body thinks you're getting far less than you really are. Try blending full fat milk with some protein, ice, and fruit (and if you're feeling particularly adventurous, a raw egg or three), and having that once a day to push up your calories.
Good fruits to blend include bananas, berries, and pitted plums, peaches, and apricots.
1) eat real food - more vegetables, moderate meat, moderate fruits, less grains, less sugar, less vegetable oils.
2) exercise - moderate intensity cardio, sprinting, heavy lifting, dedicated stretching and mobility.
3) live - relax, de-stress, meditate.
Disclaimer: I'm not professionally qualified to make any formal recommendations. I've just done my homework and I'm my own guinea pig. All of my data, unless otherwise cited, comes from a sample size of n=1 (me).