grammar - How to change a passive sentence having transitive verb to active voice?

Part of the problem is the difference between open and opened. The first is an adjective meaning

not closed or barred: the door is open

The second is a participle, a verb form

to move (as a door) from a closed position

As a verb it can be active

He opened the door

It can be passive

The door was opened by him

Using your original sentence, the better form would be

The door is open

In this case, the verb is does not really convey active or passive. Rather, it is a state of being. There is no action. The predicate adjective makes sense.

If you want to emphasize the action, you need an actor.

The door was opened by him

or

He opened the door

In these cases, opened is part of the verb conveying the action of opening rather than the state of being open.