The trouble is that the subject and is implied: "(You must) sit down!". . .but there is no object. If it were "Sit down on the chair!" I guess you could say. . . The chair must be sat on by you! But I would never, ever say something like that. . .it just sounds awful to my ears. Also. . . I wouldn't use "cum" in an English post. . . it's the same word as a sexual term in English. Google it. I knew what you meant so it didn't bother me, but it's not a good mistake to make. It's the kind of thing I'd want someone to tell me, if I was making a mistake like it in German or French.

Intransitive verbs can't be used changed into the passive, because by their nature they have no object. The only way you could base a passive construction on an imperative such as 'Sit down, children!' would be something like 'The children were told to sit down'.