We can't add plugin for free hosted wordpress mu blog, we can only use plugins that the webmaster add to their wordpress mu web.
You can use something like rss2blog, script, you have to host at your hosting, and the hosting need php feature fsockopen and curl, usually free hosting limit or disable this feature.
Rss2blog or any script like those will input data from internet and rss feed, then post it to your blog and ping many services.
Note, for some new version wordpress mu we have to enable xml-rpc feature at settings->writing