Is she just small for a boxer or is she underweight? What are you feeding and what have you tried in the past? I personally don't like to add anything to a food just for the sake of enticing them to eat unless they are sick. I add things to my girls food as additions for health reasons. We add fruits and veggies. There are things that you can add to food to help with weight gain.
I would make sure to find a food that your dog loves and see how that goes. Or perhaps feeding more meals per day, how often do you feed now? Have you tried dry food and canned? Sorry for all of the questions, just trying to get more info.