Healthy ways to add a bit of extra calories to your food without making it unhealthy include things like:
Add skim milk powder to your drinks (smoothies & normal drinks), mashed potatoes, soups, etc. It contains a lot of protein but not much fat which is good for lean weight gain.
Dress your meal with a splash of olive oil or add light grated cheese
Make salad dressings from oil/vinegar instead of using bottled stuff
Add extra oils from veggie sources - like an extra layer of avocado on your sandwich, olives in your salad, etc
Try to have some protein with your breakfast, that might mean a poached egg or small can of tuna with your toast, or add some yoghurt to your cereal, etc