You can add freshly expressed breast milk to refrigerated or frozen milk. However, thoroughly cool the freshly expressed breast milk in the refrigerator or a cooler with ice packs before adding it to previously chilled or frozen milk.. If I warmed a bottle of breast milk and my baby didn't drink any (she went back to sleep), can I put it back in the refrigerator and offer it again? Yes. You can offer it again within the next two hours.