I don’t believe you can convert to Zoroastrianism...
Sure you can. You just can’t do it in the bigger zoroastrian countries due to religious persicution (my guess).
Any religion that would refuse people from coming into it is never going to leave being a “Cult” and will then likely die out very soon.
They believe everyone is supposed to remain in the religion of his birth. They don't even feel the need to win Iran back from islam. Everyone is exactly what they are supposed to be.
There is a similarity in this teaching and that of a much younger Iranian religion, bahai, which teaches that all religions are true and all "sacred texts" inspired.