All that Zoroastrians doing it would require (at least, as far as making it possible) is a crushing Persian victory over the ERE in its wars of the 500s and 600s, and maybe Islam not showing up.
Both are possible, at least. The first, the Persians very nearly had IRL, and just couldn't resist the lure of trying for Constantinople instead of settling in and finishing things in the next war. The second just requires that Muhammad have an accident or something before he gets his new religion off the ground.