Extending RSS format with more fields?

According to the RSS 2.0 Specification then:

"RSS originated in 1999, and has strived to be a simple, easy to understand format, with relatively modest goals. After it became a popular format, developers wanted to extend it using modules defined in namespaces, as specified by the W3C.

RSS 2.0 adds that capability, following a simple rule. A RSS feed may contain elements not described on this page, only if those elements are defined in a namespace."

Check out the article Extending RSS 2.0 With Namespaces which shows you how to do this. An example from the article shows the author adding some custom blog fields to their feed:

 <rss version="2.0"     xmlns="http://backend.userland.com/rss2"     xmlns:blogChannel="http://backend.userland.com/blogChannelModule"> <channel>  <title>Scripting News</title>  <link>http://www.scripting.com/</link>  <blogChannel:blogRoll>http://radio.weblogs.com/ ... /file.opml</blogChannel:blogRoll>  <blogChannel:mySubscriptions>http://ra ... /file.opml</blogChannel:mySubscriptions>  <blogChannel:blink>http://inessential.com/</blogChannel:blink>  .  .  . </channel> </rss>

You can extend the RSS message with any elements that you want, and the way that the RSS readers distinguish the standard elements from extensions is that the extensions is in a namespace. That way a standard reader can easily read the standard elements and ignore the extensions.

http://cyber.law.harvard.edu/rss/rss.html#extendingRss:

A RSS feed may contain elements not described on this page, only if those elements are defined in a namespace.