I have been topping up with formula milk (enfalac) since baby's birth. He has no prob drinking both breastmilk and formula milk. He does prefer breastmilk as it is not so thick. But now that I have stopped pumping, he has adjusted to drinking formula milk only.
you need to try and error for formula milk until you found which one suitable for your baby. maybe you can try find similar taste like breastmilk.