@pluralistic @skaverat You should be able add a rss url the same way that you add any contact to follow. (I don't use Mastodon, so I can't give a step-by-step.)
This thread from @feditips might help:
mstdn.social/@feditips/1083579…
eta: It seems that stock Mastodon doesn't support that, which is a shame. Since almost everything on the Fediverse is available as a RSS feed, being able to follow any RSS feed completes a powerful feature.