The best way to add more calories without adding more carbs is by using fat. Cook your meats and eggs in butter or olive oil, use full-fat salad dressings on your salads, use mayo and make tuna and chicken salad. Put plenty of butter on your veggies. Choose fattier cuts of meat and poultry.
A day or two at 1,000 calories or less isn't going to do permanent damage, but you really don't want to stay that low for long.
You might also find it more helpful to get the calories in if you eat several small meals instead of 2 or 3 larger ones.
You should also be eating more veggies than you're eating. You do know that you can subtract the fiber from your total carb count, right? You should be eating 20 grams of carb per day after the fiber is deducted.