"I am also standing by this door."
Translation:Én is ennél az ajtónál állok.
The word "is" comes right after what it refers to. So, if it is "I, also", then "én is". If it is "at the door also", then "az ajtónál is" . Etc.
In order to use the definite case, it is not enough that there be a definite object (door, etc. ) That object must also appear in the sentence as a direct object. That is not the case with the sentence here.
Whether you use definite conjugation or not is a matter of whether the direct object is definite, i.e. the object in accusative case, i.e. the object with a -t suffix. Since áll is (pretty much) completely intransitive, you won't have much of a chance encountering állom.
But you can paint this door if you like: "Én is ezt az ajtót festem." -t suffix and definite conjugation, there you go.