How to add multiple categories to one rss feed?

Question

Feeds

All the best to all.
Site on wordpress.
It is necessary to set up an Rss feed for sending in letters. But not all the headings of the site should be included in the feed, but only the favorites.
I did this by eliminating unnecessary ones.
Everything pulled up as it should.
But the site also uses a plugin that distributes articles from headings to Zen news channels, rambler and others.
So this is where the problem started.
For example, there is a heading "news".
I need to remove her from the mailing list. But leave in the zen channel.
If I write an exception by id in functions.php, the category is excluded everywhere.
How can I prescribe pulling up rss in the mailing list of a letter not by the method of exclusion. What about the inclusion method and is it possible at all? Maybe there is some plugin or service? Thanks in advance to knowledgeable people.
some such code
site.ru/category/news,pogoda,sport/feed
but only working)