I have a RSS feed in an XML file at the root level. I have a php script that adds to the xml when ever i want to add to it. Is there a way to have wordpress add to that RSS feed every-time publish a blog post?
Yeah, that'd be no problem so long as you can parse the XML and then append nodes to it (XML docs are not as straight forward as, say, doing a file_put_contents() to append to the doc).
One way to do this would be to hook onto the edit_post action and do your update in the callback function.