Yes, you can mix if the milk expressed earlier is at room temperature and expressed not more than 8 hours ago. However, if the already expressed milk is in a cooler with ice packs, you will need to cool the freshly expressed milk first for about 30 minutes before adding it to the cooled one. This is to prevent the already cooled milk from warming up and then cooling again which might affect its freshness. This also applies when you are storing breast milk in the fridge at home.