I guess you would have to do some changes in your recipes to do that. You could change the parse_feeds() function to only get the articles that were added since your last access. This would require you to store the date of the last access somewhere, or you could extract the date of the file. Then you could compare the article date with the date of the last access and only retrieve the newer articles.
Here is a description of the parse_feeds function:
http://calibre-ebook.com/user_manual...ownloaded-html