I agree with elusive, fats are the best way to add a lot of calories to anyone’s diet. You only need to add 100g of fats to get 1000 calories, as compared to 250g of protein or carbs. That being said, 1000 cal of any macronutrient, particularly fat, will probably seriously unbalance your diet.
If you are having trouble gaining lean mass, the first place to start is protein. You need at least 1g for every pound of body weight (but 1.25g to 1.5g is not unusual or harmful if you are healthy and drink lots of water). Once you have your protein intake in line, you can add fat or carbs. Good fats (unsaturated fats, especially the Omega line) are an easy option. A few tablespoons of fish or flaxseed oil will quickly add a lot of calories to your diet - I keep a bottle of fish oil in my desk at work.
You should also be eating 5-6 meals a day instead of 3, and eating a big breakfast. If you are only eating 3 meals now, you may find that eating the same quantity of food more often (every 2-3 hours) may help even without a lot of extra calories.