Include starchy foods in your diet. White rice (plain), potatoes, yams, squashes, and the like. Eat more fats. Diversify your animal proteins. And an 18 BMI isn't "too low", IMO, you just don't need to go any lower.

EDIT: I asked a similar question last year Hack my "Over" weight loss, you may find some helpful suggestions!

Include starchy foods in your diet. White rice (plain), potatoes, yams, squashes, and the like. Eat more fats. Diversify your animal proteins. And an 18 BMI isn't "too low", IMO, you just don't need to go any lower.