I dont know anything about low/no carb diets but try getting in calorie dense snacks, its about the only way I know that I can get more calories but still eat healthy. some things I have used in the past are :
all sorts of nuts and nut butters
cheese, cream cheese
avocados
butter, cream
dark chocolate
bacon
olive oil in salad dressings
mayonnaise (for me, one of the major food groups)