The question tells almost all. I have a Pi Zero W at my balcony. It's connected to my home router through wifi. For now it's running lighttp server, taking a picture every minute and measuring temperature and making a nice plot of last weeks temp measurements and showing them when asked.
Problem is that my home router gives a terrible wifi signal outside house (in balcony it works but 10 meters from there it dissappears) and I was thinking to turn that Pi Zero W to a wifi router because it is already out there and so I would be able to use my own internet connection when sitting in the park out there. I can always buy a usb-ethernet adapter and a long cable (10 m or more) and make it happen (I hope). But I have much nearer a small debian server running inside house just on the other side of the wall and would need only 2 m usb cable if I connect that Pi Zero W directly to it. That debian server does not have free ethernet ports but it has free usb ports.
As far as I understand it should be possible but I have had no luck in finding instructions how to do it. So any hints and links would be highly valued.