To add more oil or cornstarch, I find it helpful to have a little dish of either the oil or the cornstarch next to where I knead the dough. If I decide I need more of one, I place the dough in the little dish to gather a small amount and go back to kneading, much like if I were preparing pizza dough and needed a little extra olive oil or flour. I then continue to knead the dough with a little extra of either ingredient until I have the consistency I want.