How can an immaterial being impart material substance when it doesn't possess it?

Simple physics. Matter and energy are interchangeable (which is why atomic bombs explode). Thus if the energy is available...it can be converted to matter. It's actually a very interesting connection between the primary forces in the universe (electromagnetism, gravity, weak nuclear force, strong nuclear force) quantum entanglement and chaos mathematics that would allow this to take place. You see since all these forces are connected and since at the beginning of the universe there was only energy created in the big bang, energy which was converted into matter, all energy and matter in the universe are connected on a quantum level. That being said that makes it possible for matter and energy to be easily transformed one to the other.