Yes you can mix it. Pumped milk is good in the refigerator for 5-7 days, in a regular freezer for 3 months and a deep freezer for 6 months. Even if you need to mix milk from different days that is fine, just make sure if you're mixing refrigerated milk, make sure they're both the same temperature first. Meaning, if you pump in the morning and then again in the afternoon, the afternoon milk needs to get cold before mixing it with the morning milk. I had to buy little freezer bags and tubes for milk storage, I got them at Babies R Us. Good luck!!