Question

Someone has knocking at the door
?change into passive voice

Answer 1

The door has been knocked by someone.

Answer 2

In a sentence with an active voice, the subject performs the action which is denoted by the verb. In other words, the subject is the doer of the verb's action. Whereas in a passive voice, the subject is not active but by the verb is acted upon.

Therefore the above sentence "Someone is* knocking at the door" in passive voice would be written as:

The door is being knocked at.

(*auxiliary verb used in the question is incorrect, it will be 'is' or 'was'.)