Eat em with anything else and it's just extra cals on top, since you're going through the digestion process anyway.
To be fair, if you eat a LOT of fibre with other food, it can reduce the calories you get from the meal because so much of the surface area of that meal never gets digested.
But I'm sure that's not what the OP meant by negative calorie foods.
I blend stuff into soups all the time. Makes for a tasty, thick, satisfying soup and as an added perk, hubby doesn't like the texture of some veggies, such as caramelized onion. Blended, you still get the taste but not the texture.