Hey. While freezing breast milk is ok, you would be best only to pump enough for a few days at a time and not freeze it. It looses some nutrients when frozen. Also, when pumping you want to be sure to completely empty each breast. A lot of the time when women pump they will do so after nursing and breastmilk comes in two stages, first it is formilk that has all the nutrients, and then hindmilk that has all the fat. The baby needs both. When you pump, even if you only get a little at a time it will have both parts. which is why it would be best to either mix two whole bags together or none at all- meaning dont use like half a bag. Try to make bottles for either the amount the baby drinks or less, you can always make more if need be. NEVER warm it in the microwave.
Mixing it with formula is not recommended. You would be better off warming two bottles, a formula bottle and a breastmilk bottle. Also- NEVER EVER EVER EVER give your baby cows milk- not even mixed with your milk. This could cause internal bleeding and give your child many lasting effects- even if you THINK your baby is ok now. It can mess up their digestive system permanately (kind of like giving cereal too soon or putting it in a bottle). It's very dangerouse.
So pump what you can. if you want to substitute formula that's fine, but dont mix them together. I know I used to work for a family (I am a nanny) and the mom had a hard time pumping so durring the day the baby got one breastmilk bottle and the rest formula. The mom nursed him in the morning and when she got home. This might be less stressful then trying to pump enough- I know how hard that is!!