C.G.
I wouldn't worry about mixing it... or pedialyte if the baby would happen to vomit (unlikely) from the milk- breastmilk is far superior to pedialyte for dehydration. Most likely if she has a reaction it will be a rash or diarhhea type things nad if she hasn't ever reacted to any other dairy product (or formula if she has had it) she probably will not react to milk. If you do want to mix something I would mix breastmilk and cow milk to help her transition the taste and consistency. I would not mix with water since that will reduce the amount of fat and calories she is getting from the milk. I have never heard of anyone mixing water and milk, honestly.
Like another poster suggested, just try reducing one feeding at a time. Often the morning and evening are the hardest to give up. Offer her a sippy cup of milk instead and avoid pumping/nursing at that time to signal your breasts not to make milk for that feeding anymore. Once you are both comfortable with that you can eliminate another feeding until you are done. Good luck.