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You can add multiple sitemaps in Google Webmaster Tools, that's not a problem. So you could, I suppose, add a sitemap of just your new pages. In my opinion though, I think you should just generate a new one at http://www.xml-sitemaps.com and upload and resubmit it. That would work just as well as a part sitemap. If your site is really large though, maybe the part sitemap is the right answer, but going forward you'll have to remember which bits appear in which sitemap so you don't have overlap or accidental omissions. Hope this helps.