To be in the active voice, the sentence needs a definite subject. That is, you need to say who opens the door. If you don't know who does it, you could use:
Somebody opens the door.
Sentences like "the door is opened" can be parsed in two ways: as passive voice present tense, or with "opened" being an adjective and "is" a copular verb. This parsing can be made unambiguous in several ways; for example "the door is opened by remote control" can only be passive voice. One way to put this sentence into active voice is "you open the door by remote control".
For the second parsing, it's not in the passive voice, and in this case, the meaning is the same as
The door is open.
The verb "is" cannot be put into passive voice, because only transitive verbs can be put into passive voice.
The question touches on several issues. Stated as it is, there's no single answer.
However, many of the issues touched on are fairly well understood.
First, terminology. Passive refers to a syntactic process only. It does not refer to meaning.
Consequently one cannot "express the Passive voice by means of the active voice". Or by any means.
Passive is not "expressed". A Passive clause is determined by inspection. If a clause has
1. a be auxiliary verb, followed by the past participle of the main verb, and
2. a patient subject that could be the object of the active verb, and
3. an optional by-phrase agent that could be the subject of the active verb,
then it's Passive. Otherwise, it's not Passive. What it means or doesn't mean is irrelevant.
That's so you understand what I'm talking about, which is variation in Subject and Object.
Passive is just one of a number of ways English has to vary what nouns appear as Su and DO.
(which I suspect is what is meant in the original question, or I wouldn't answer it this way)
Two ways in particular are mentioned in the question.
One way is what Colin Fine points out is called the Middle construction, or alternation.
It's the first topic taken up, on p.26, in Beth Levin's book English Verb Classes and Alternations.
It has a lot of quirks; as Levin puts it,
The middle construction is characterized by a lack of specific time reference and by an understood but unexpressed agent. More often than not, a middle construction includes an adverbial or modal element.
Some other Middle examples (asterisk * before a sentence indicates an ungrammatical sentence):
- This book reads easily = This book can be read easily =
Unspec can read this book easily
- This book sells fast = This book sells itself =
Unspec can sell (many copies of) this book easily
- This dress travels well = It is easy (for
Unspec) to travel with this dress
but
- *French fabrics adore easily ≠ It is easy (for
Unspec) to adore French fabrics
Another way to vary Su and DO is to use a present participle, instead of a Passive:
- The bridge is still being built = The bridge is still building =
Unspec is still building the bridge
The second one of these is an areal variant, dating back to an earlier construction.
In some areas of the Anglophone world, one might even say The bridge is still a-building.
This is similar to the areal usages of present and past participles with need:
- This car needs washing = This car needs washed =
Unspec needs to wash this car
the second example above is, again, areal.
Best Answer
You're confusing things. In order to transform a sentence from the active voice to the passive, it must contain a transitive verb and an object. The object becomes the new grammatical subject (called the patient) and the former subject becomes the agent following the prepositions by or through, or is simply left out:
Intransitive verbs can never be in the passive voice because there is no object to become the new subject: they do not tranfer action from a subject to an object, so that action can't be recast as patient and agent.