I think it's a good idea. I want to make it possible for people to administer their own feeds, without creating spam, and I think having moderators able to add feeds to communities they moderate would be a good stepping stone.
How about adding a feature to let people write:
@[email protected] add https://some/feed !some_community@instance
@bot.rss.ponder.cat remove https://some/feed !some_community@instance
in DMs or comments, and control their subscriptions directly? I'm still not sure about letting them do that to create communities on rss.ponder.cat, but for communities they already moderate, it sounds like a good thing.