I am breastfeeding my seven month old daughter, and even though she is very small (2nd percentile weight for height), my doctor hasn't suggested supplementing with formula. When I give her a bottle of expressed milk, I do add rice cereal and olive oil to add some extra calories for her. Now that my daughter is beginning to eat some solid foods, I'm adding oil to those meals as well.
I think if you have a good milk supply, your baby will get as many good, rich calories from your breastmilk as she would from formula. There are many ways to increase your milk supply, but as long as you are getting enough calories yourself, your body will naturally provide as much milk as your baby will take. I pump at night so my husband can feed our daughter her nighttime bottle, and I am amazed at how my production varies based on how my daughter has been eating.
Ellie was sick for a couple of weeks and I found that after a day or two of her disinterested feeding, I began producing less milk. Once she was healthy and back to eating normally, I started pumping more milk at night. It's very much a supply and demand thing! That said, if you do find you are not producing as much as you think your baby needs, you might try pumping in between meals to stimulate more production. And if that doesn't work, I'm sure formula will add the calories that your daughter will need.
Best wishes!
-- Jenica
mom to Abby (2, w/o CF) and Ellie (7 months, with CF)