Oh, all you have to do is first place it as a disabled sprite(preplaced) enable and order it, so it's stacked, and then do the cloak trick on it like you would any other unit.
UndeadStar -- https://staredit.net/topic/18961/ <= wow, if there wasn't a capital letter on almost every word in the title, and a link in the signature, I may not have guesses it's an AI bot. I don't know much about map editing, but the content may even be relevant enough to be kept.