Hey Threebiz,
Introduction:
Having a sitemap (XML) is good for diagnostics and helping searching engines - to discover your content. It's something you should consider adding to your Google Webmasters Tools area as well. It will give you stats about pages it finds, and the index count, any errors, etc.
On point:
You shouldn't do this "addition" manually. For example, if you install one of the popular XML sitemap plugins for your WordPress (assumption made) installation - it will auto-create an entry for each new blog post - into the XML sitemap file in the root of your server.
One more thing:
If you have a highly trusted website - and a good organization of your links/navigation, etc - and that all pages are reachable by a link - a sitemap XML is just a bonus. Some will not even worry about it, just FYI.
Hope that helps! - Cheers, Jon
P.S.
On a related topic - some will go out and use a service like pingler.com to have your new blog "be discovered". It's something you might do on a brand new website, but not after that. No spamming!
