Don't freeze the milk- use it now. If you can't get enough for a full bottle, either combine the milk from 2 pump sessions or offer pumped milk first and then finish the feeding with some formula. (You don't want to mix breastmilk and formula in the same bottle because the storage guidelines are different: unfinished formula must be discarded, breastmilk can go back in the fridge. If the formula and the breastmilk come in different bottles, there's no chance that you'll be sending your hard-won milk down the drain at the end of a feeding.)