You can check in your rss client documentation if it supports socks5 proxy and maybe even resolves onion adresses. If it does both - just slap that .onion in your urls. If it doesn't resolve onions but does support socks5 you could map that onion to some IP address. Add a line to /etc/tor/torrc:
mapaddress 10.40.40.10 fqcoretoclxwqpy3dwktkskmwywrcktgnpviesveq6vrjhqhdc6y4fid.onion
Remember to restart tor daemon. Example configuration for newsboat. ~/.newsboat/config:
use-proxy yes
proxy-type socks5
proxy localhost:9050
And ~/.newsboat/urls:
http://10.40.40.10/rss_tor.xml "News"
If your rss client doesn't support socks5 or you don't want it to run through tor all the time you can save .xml file locally every time you run your rss client (for example using an alias) and point your client to that file. Example for newsboat, ~/.bashrc:
alias news='torsocks curl http://fqcoretoclxwqpy3dwktkskmwywrcktgnpviesveq6vrjhqhdc6y4fid.onion/rss_tor.xml > ~/.newsboat/fqcor.xml && newsboat'
And ~/.newsboat/urls:
file:///path/to/homedir/.newsboat/fqcor.xml "News"