Article Generator permits to generate planet-type sites.
Starting in Tiki5, you can now not only show & aggregate RSS feeds, but you can import them as native articles in Tiki, where you can comment, vote on, tag, categorize and redistribute via RSS.
ensure the articles feature is enabled
go to tiki-admin_rssmodules.php
create an RSS feed
Click on "Article Generator"
As a related topic, Calculations can be used to create custom sortings based on article aging and user ratings (Digg-style).
Important Caveats:
RSS feeds are only refreshed when someone clicks on the icon or when there is a RSS module somewhere (could be through smarty {rss..} tag. So if you want to auto refresh feeds (articles are only generated on refresh), you would need to put a rss module somewhere that is loaded on normal user activity, or write your own cron job.
At the point of configuring the feed in tiki-admin_rssmodules.php, it gets loaded into the system. All existing items that are being served by the feed gets loaded, but articles are not created for them because the article generator is not set yet. Then you click on the Article Generator icon and set the article generator settings. From that point onwards new items in the feed become articles each time the feed is refreshed (which is either when someone clicks the refresh icon or the feed gets loaded in a rss module). But only new ones. There is no way to retroactively make articles of those items that were in the feed when you last refreshed unless you manually remove them from tiki_rss_items database table (for now anyway, until someone makes a feature to manage past rss items).