Before you mix the two, understand that breastmilk is on a "supply and demand" formula. By introducing formula, you are risking supply problems to your breastmilk supply, especially this early on.
Another suggestion is to feed more frequently during the day, and even allow her to "cluster feed" before going to bed at night. As time goes on, she will start sleeping longer through the night.
Speaking from experience, my first child nursed every hour non stop through the day, and then every 2 hours at night. The only way I got sleep was by cosleeping, and then discovering how to nurse him while laying in bed. I worked full time, and pumped during the day at work, and even had some early supply problems to deal with. It was a tough 6-8 weeks in the beginning, but if you can get through this stage and keep it to only breastmilk, then in the long term, you and your daughter will be much better for it.
Adding formula will not make your daughter sleep longer, and you are risking even less sleep on you and your husband as her system is not equipped to digest formula as well as the breastmilk.
At this stage, you should try to grab sleep whenever you can, and if the baby's sleeping, it's a great chance to get some sleep for yourself. That is the only thing that saved my sanity in those early weeks.