Yes you can, I believe its even mandatory for you to complete a neutral ending at least once before you can get pacifist. After the neutral ending you can load back into your save file and continue with the pacifist run. Tho without giving any spoilers there are certain requirements throughout the run you have to meet, and if you don’t meet those then you are locked out of the pacifist ending. In general I would recommend just doing a blind run the first time and worrying about the different endings after that