Yes u can but then u will have to use it within 2 hours at room temperature otherwise u can store pumped milk for 4 hours at room temperature

If your baby is able to tolerate formula feeds,then there is no harm in mixing both

No not at all Formula feed is a chemical. So need to be used as directed ie 1 scoop in 30ml water only. Not to be mixed with food, breast milk or dairy milk. Can always give breastfeeding first. If child is not satisfied then give formula feeds. If child is gaining wt like 150 to 200gm in a week and passing urine more than 8 times a day, then avoid even 5% of formula feed. Not required then.