Interloper

[Hana]

A stray cat [maybe] has made it's way into our home.
It's been sneaking in and out since three days ago, but I know he has been sleeping outside in the nights around my home stalking/howling after our lady cat [spayed, unfriendly].

I think it is certain companionship is all he is after, all he tries to do is sit near her?

She is nearly 8 years old and hasn't lived with a cat in 4 years, and she is very hostile towards other cats.

It is a problem since I need her to tolerate him while we try to find his owners, so I've tried to keep him in a spare room.
Only he is very smart and knows how to turn the doorknob

Does anyone have advice on how to get them to get along?