Q&A: Mixing Fresh And Frozen Milk?
Can I mix freshly pumped milk with frozen milk?
The Centers for Disease Control and Prevention recommend that you don't add fresh breast milk to previously pumped and frozen breast milk, saying it's "best not to mix the two." The problem is that the new milk partially thaws the frozen milk for a short time.
Other experts say it's okay to combine fresh milk with frozen milk but only if you cool the new milk first in the fridge and there is less new milk than already-frozen milk.