I first want to say congratulations on being able to breastfeed for this L.! That's an awesome accomplishment in and of itself. I had to stop breastfeeding my son when he was just over 2 months because of some medical problems he was having.
You really aren't supposed to introduce milk until after the 1st birthday. Milk products are fine after 9 mos (yogurt, etc) but milk itself should wait until after the birthday. My son's pediatrician told us that milk, eggs, peanuts and shelfish need to wait until after the 1st birthday.
If your daughter will drink it without a problem, then there is no need to mix it. Some children will refuse and that's when its recommended to mix it with breastmilk over time, slowly increasing the milk and decreasing the breastmilk. Some children are also picky about the temperature of it. If she's not having any of those issues, I say there is no need to mix (but it wouldn't hurt if you wanted to mix it to use the milk that you have frozen, which by the way, in a deep freezer remains good for 12 mos).