Susan,
I always went by having the milk the same temp.  So, no warm to cold, no warm
or cold to frozen.  If the mom expresses milk at 8:00 and puts it in the
fridge, and then again expresses as 11:00 - she should first refrigerate the
milk and when it is cold she can add it to the first.  If the mom freezes the
milk, she can freeze small amounts separately and then add the frozen pieces
together in one storage unit.

Dori in NNJ