hmm...if you are gonna use the pupil as a seperate geometry, i think it may be possible through some adv. mel scripting to have the pupil move along the surface of the white eyeball keeping it's same volume area.
Otherwise, have you thought about just painting a black pupil on the white eye ball? since it's just a plain black one colored pupil.
9 months into Maya...and still learning...
0 experience with any other 3D programs