Learn English – Adverbial phrase in passive construct

There is nothing grammatically incorrect about

His room is being painted by John.

However it doesn't carry the exact same meaning as

John is painting his room.

The first (passive) means that John is painting someone else's room, where as the second (active) could mean that, but it's more likely that it means that John is painting his own room.

You clearly know how to form the passive, but the change that needs to take place is that you need to make sure the same person is doing the same thing to the same object as in the active.

If we assume the room in "John is painting his room" to be John's room, which is most likely without further context, then to turn it to a passive construction you need to start by specifying whose room it is: John's room.

Next you make the action of painting passive, which you did correctly: is being painted.

Finally you specify who is doing the painting: by him.

John's room is being painted by him.

The last part (by him) can cause the inverse ambiguity of the original (active) sentence, i.e. we assume the him refers to John but it could refer to someone else. To completely remove the ambiguity you could change the last bit:

John's room is being painted by John, himself.

But usually the given context removes ambiguity.

Andrew Leach's explanation is sound as far as how you might go about doing this, but the answers all sound awkward and goofy. I think this points to a bigger point that you shouldn't do this.

Consider the reasons for using passive voice. The whole idea of passive voice is inverting the emphasis between subject and object, so it just doesn't make a whole lot of sense to do in imperative sentences.