Nothing is impossible in dreams, or at least that's what I say. I do think that if I were to summon a scientist in my dreams and ask him/her about, say, quantum physics (something I know nothing about) that I would be able to learn something about physics.

Though I think many people would agree that if you practice something (say, math) while in a dream, then you would get better at it, as if you did it while you were awake. While in dreams, your body does not physically respond, but your nerves still send the signals, so if you practice the piano while in a dream, learning the piano in waking life would be much easier.