Not sure what you are asking here? I guess you aren't asking for RSS feeds app like Heartfeed, Newsify, Feedly, TechPort that support RSS feed and most of them also support manually adding RSS feeds in those apps.
Reeder will try and find it for you based on the link you try adding. There's also an action in Shortcuts that does the same thing.
EDIT: Or an exported list of them (you posted on the News Explorer thread)? In that case you put the OPML file in Files via iTunes/Finder (you can also just email it to yourself) and you get to it via the Share Sheet or the app itself.