I breast feed my baby, but is it ok to add formula powder to her food?

My baby has been on just breast milk for 7 months but she is in the 10th percentile for her height and weight and I'm worried shes not getting the proper nutrients. Also her iron was low when we were at the doctors. So I tried giving her a bottle of formula yesterday and she spit it out. I want to know if its ok to add formula powder to her food? I will call the Dr. but until then any suggestions?