If you are displaying the feed entries on your page, you would be using HTML. If you use HTML5, you may use Microdata. If you can’t change the output on the server-side, you’d have to add the Microdata with JavaScript (if you are interested in search engines consuming your Microdata, then this would probably not work for most of them).
If you link to the feeds from your page, they are probably in XML (Atom, RSS, …). Microdata can only be used with (X)HTML5.
If you want to use the schema.org vocabulary, you can use Microdata, RDFa Lite, JSON-LD, ….