Can chilled breast milk mix with expressed breast milk immediately ?

You should only mix at the same temperature, see this article from Kellymom which is a known excellent resource when it comes to breastfeeding http://kellymom.com/bf/pumpingmoms/milkstorage/milkstorage/ You can see the comment "Avoid adding warm milk to a container of previously refrigerated or frozen milk – cool the new milk before combining"