make formula separately with water and then mix with the amount of bm you want. DO NOT just add to bm. Also don't mix the 2 together until you are ready to feed.
Make the formula according to the directions then add the breastmilk. If you don't use water it will be too thick and concentrated. I would say phase it in, like 3/4 BM and 1/4 formula for a couple days, the 1/2 and 1/2, then 1/4 BM and 3/4 formula. You could switch cold turkey and LO will be fine, but it may ease the transition a little.
Re: Adding formula to breastmilk
My first instinct would be to make the formula like you normally would (using water) & then mix breastmilk into it.
DS2 August 2012