Can you build an afocal zoom system with only one stage. It looks like you would have to use more than three lenses. You would probably have to have all but one of the lenses be fixed; but it might be possible to have more than one moving, as long as they are all moving the same distances (i.e. they are on the same stage).
If not; can you prove that you can't do it?
Some helpful links: http://en.wikipedia.org/wiki/Zoom_lens http://spiedigitallibrary.org/oe/resource/1/opegar/v36/i4/p1249_s1