To write this response, hopefully you have read the poem. In the poem, the weary traveller goes to the house, and addresses a host of supposed ghosts or spirits that haunt the house. He stands there and before he leaves, he mysteriously states, "Tell them I came, and no one answered, that I kept my word." So, the traveller is coming to fulfill some sort of promise that he has made; he is relaying information that he has fulfilled a mission of some sort that he has promised to accomplish.
So, in pretending that instead of strangers who "stood listening in the quiet of the moonlight," never answering the door, actually open the door to the traveller, and address him, you must use your imagination. First of all, I would imagine what exactly the traveller had done while away; what promise did he make? What did he set out to do? Did he go on a quest of revenge? Was he a hired assassin? Was he fulfilling some terms of service that would set him free because he was a slave or a servant to the household? Was he on a secret mission for some covert rebellious group? Was he a spy? Was he fulfilling a romantic quest? Decide what task he is returning and reporting from.
Once you have decided on what the traveller's mission was, decide what the strangers will look like. Will they be ghost-like? Will they look like real people? Will they be old, young or both? Then, what will they say in response to the information that the traveller gives them about his quest? Will they be upset? Accepting? Ominous? Threatening? Send him off on some other quest?
Then, to make the entire atmosphere mysterious as you write, have the wind blowing, dark shadows being cast, and eerie noises. Also, don't explain everything. Leave some element of mystery or questioning to the encounter, so that the reader is left wondering what the entire story is; that is what makes a good mystery.
I hope that those suggestions help; good luck!