Wonderful problem to have!
Here are a few tips:
Load a tbsp. of nut butter onto a couple of rice cakes and eat them after or in between a meal = 150kcals; double the dose, if you can, and you've added 300kcals to your meal and you will hardly have noticed. Do this 1-2 times a day, more if you can.
After dinner at night, empty 50g cashews into a bowl with around the same volume of giant raisins. Snack through them after dinner and you've added nearly 800kcals, again you will hardly notice the bowl being drained.
Another option is mixing up shakes in a blender a la Dave Draper et al. The point is adding calories dense foods like nut butter, full fat milk, banana, coconut oil, etc. are all simple ways to crank up calories in a 500ml drink, which can be easily downed (even before bed).
A carb tip is to chose rice over potatoes. It's far easier to eat 100CHO from steamed rice, IMO, than the same macros from potatoes. As an aside, rice is really flexible and can be used for sweet purposes too, using syrups, honey, nut butter, etc.
Finally, watch your fibre intake. Green veg, etc. is great for you and shouldn't be shunned but some folks underestimate their fibre intake and, as a result, you do literally feel full (because you are). (Oats can do this in the morning too, although they are a good calorie source. Again, I'd argue the same macro quantity in rice or quinoa is far easier to consume). For this reason, another tip is do the reverse of dieters and consume your greens last. Start meals with aggressive tastes; salty meats work well, with your carbs, and your appetite is more likely to flourish.