That cannot be done inherantly with the spell starfall. The actual spell doesn't actually cause anything to "fall" - the star model animation actually shows the falling effect (same with the meteor from infernal, or shards from blizzard). To use blizzard as an example, if you changed the graphic to something like explosions, it would look like ground based explosions, not falling explosions.
To do a falling effect, you would have to use flying units and trigger them to lower their altitude over time. Of course, the rocket model probably wouldn't be facing down perfectly, so it'd look pretty messed up, though I'm sure if you looked hard enough you could find something on one of the models sites for a nuke or something.