One of the main reasons for mixing with formula or breastmilk is so that the taste is familiar to a new solids eater. Rice cereal itself has no taste, so mixing it with something familiar can help with the transition to solids. Even if you don't pump, it really doesn't require much ebm to mix with cereal, especially at the beginning. You can hand express the tiny amount you need right then, if you want. That's what I used to do.