Opinated breastfeeding mothers?

I called a breastfeeding hotline about this question a couple days ago. She said to pump your breastmilk into a bottle and add a little formula to it. Over a period of a few days add a little more formula to the breastmilk bottle and eventually just do straight formula. If your transition too fast it can make the babys tummy upset and they wont be able to digest the formula. Goodluck!