Insert Featured Image from FTP Uploads

Hi! I have a question. I do not want to use the MediaLibrary to handle my image includes and uploads, because I have to keep looking into all month folders to see what is where, so I have my own system of folder management and I upload images there using an FTP client. This is fine for inserting regular images into a post, because in the URL tab, I can type the link and the image shows up. But when I try to set featured image (it is already activated), there also I get an URL tab, which does not function in a similar fashion! I have two questions, which are in fact very similar to each other.

First, how can I code in a featured image without using MediaLibrary at all, if it is in my Upload folder but not in the WP database?

Second, if I have already inserted an image into my post by typing the location manually, how can I set some option to use the same image as the featured image for that blog post? Is there a shortcode, or is there a plugin that would provide such a feature?