All calories come from one of three things:
1. Fat - 9 calories per gram
2. Carbohydrate - 4 calories per gram
3. Protein - 4 calories per gram
So the only way to add calories to your diet without adding carbs or fat would be to increase your protein. If you choose very lean protein sources, you won't get too much fat with them.
You might also consider adding low glycemic carbs. These are carbohydrates that, while they are starch, have a low impact on blood sugar because they have a lot of fiber to slow digestion and prompt a slow rise in blood sugar (as opposed to a quick one provided by simple carbs, like white sugar).
You can learn more about the glycemic index here:
http://www.glycemicindex.com/