[lch]

The ability to fly during storms isn't moddable as of yet, AFAIK. A number of units which have a strong connection with Air magic (ironically, the Unwinged, even), or the Void, or Horrors, can do it.

Edit: I've been thinking it over, you can try what Sombre used to do: Copystats from a unit that has the flag and then edit it until it fits what you want. You can use the Unwinged (1674), like I wrote. That's the only unit that has the flag and doesn't already have flying.