Mixing the two is possible, even in the same bottle, but you need to be careful to follow the mixing instructions before combining formula with breast milk. Improper mixing can lead to an over-concentration of nutrients that may pose a danger to your baby.
In other words, if you have 1 ounce pumped, cool it and add only that ounce to the frozen supply. Only add cold milk to cold milk or cold milk to frozen milk. Because you never want to waste breast milk, you might consider freezing it in relatively small amounts. This way, if your baby doesn't take it all, you won't be throwing it out.
While there's nothing wrong with mixing breast milk and formula in the same container, it's not recommended simply because you don't want to waste a single drop of your precious breast milk. Formula from a bottle that your baby has drunk from must be discarded within an hour of preparation.
Mixing Breast Milk and Formula Combining Breast Milk and Formula. Combination feeding baby (Physically mixing breast milk and formula in one bottle, is discussed lower on the page) A combo of breast milk and formula is definitely a better option than ending the breastfeeding relationship altogether, as any amount of breast milk is better than none.