K2 uses Joomla's feed parser, which means you can get ?format=feed&type=rss or ?format=feed&type=atom.

If these don't fit what you need, I recommend you grab the json output ?format=json, json_decode it and then do whatever you want with the data. Much better imho.

You can't override how RSS/Atom are produced without modifying core K2 views.

As for JSON, it works just fine, e.g.: getk2.org/blog?format=json

That view is available for categories/tags/users (any itemlist) and items only. Perhaps you were testing a different view (e.g. "latest from X").

Just enable full-text inclusion for feeds in the "Content" tab under the K2 settings.

Great :)

That would be Joomla. I don't recommend so.

Why not use ?format=json in K2 URLs and simply grab ALL the data that K2 outputs? Much better compared to RSS/Atom feeds.

Don't hack core files.

Additionally, the RSS feed does not offer the content flexibility of JSON.

Instead do this simple thing.

Grab the JSON output that K2 provides, process it in a standalone PHP file, outside of Joomla, and simply produce a new RSS or Atom feed. Then use this "bridge" file's feed output to to serve the feed to your third party app.

Reading the JSON file is a piece of cake with file_get_contents and then json_decode. You'll get an object which you can then transform to whatever you like. Which do you think is easier? This option or hacking the RSS/Atom driver in Joomla?