Mixing breast milk and formula together is perfectly fine if you want to do so and adding the breast milk to the formula can help your baby stay healthier and gain more weight too.
So your baby will get the benefit of the breast milk along with the formula which can benefit your babies health in many ways.
Most people however just feed formula on some days and other days they might feed the baby breast milk separately however there's nothing wrong with mixing both the breast milk and formula together.
If you do mix the breast milk with the powdered formula you'll increase the babies calorie intake which is sometimes a good thing especially if your baby is having trouble gaining weight after birth.