How could you change a sentence having no subject into the passive voice in English ?

A sentence cannot be changed into active voice without a subject. Sometimes when a subject is not mentioned, it is assumed. For example, 'The door was locked.' This sentence doesn't have a direct object. Hence, this sentence can be converted into direct sentence by assuming a direct object. 'Someone locked the door.'

I'm not sure what your point is, but sometimes it's really interesting to think of how we can change certain sentences (without clearly stated subjects) into the passive :

- Stand up. (I was asked  to stand up)

- Open the door. (Let the door be opened)

I think it couldn't be changed, but send an example also to verify.

Would you provide a specific example? because otherwise I totally agree with Yasmeen's answer.