Creating a new feed from the page that already has one

[brygadasfm]

When I try to make a feed from the Rzeczpospolita news outlet (https://www.rp.pl) via moRSS.it, I do not have an option to select particular items from the main page of the rp.pl, I receive an automatically generated feed instead (https://morss.it/https://www.rp.pl/) which is in fact identical to the official feed from that site that already exists: https://www.rp.pl/rss_main.

The thing is the official feed is not what I would like to get: it consists of every single article that the outlet has published. I would like to be given a feed that consists only of the articles that had emerged on the main page of the rp.pl. I think I would be able to make such a feed if only the site did not have its own official feed that for now remains an only option, no matter if I access it through https://www.rp.pl/rss_main or https://morss.it/https://www.rp.pl/, they are both the same. If I were not transferred automatically to https://morss.it/https://www.rp.pl/ (and I would not be, if the site did not have its official feed) I think I would have to select specific content from the main page of the rp.pl before getting a new, original feed from moRSS.it. Now I am not able to select any particular items from https://www.rp.pl, it all works automatically.

Is there a way to create a new feed from a webpage that already has one via moRSS.it?