[Plugin: WordPress SEO by Yoast] Feature Request: Submit External Sitemaps

I love using Yoast for wordpress sites, and the xml sitemap feature is great. I love how I can use the main sitemap link with webmastertools and all the sitemaps will get submitted.

One feature request I have would be the option to add a external xml sitemap that get listed with all the yoast sitemaps. Why would this be useful? Well for one site I use wordpress, but also use tumblr for another part of the site. So if I could just input the tumblr sitemap via your plugin, the sitemap would automatically get submitted and updated with the yoast sitemaps.

Example of tumblr site url:
http://sitename.tumblr.com/sitemap.xml

http://ww.wp.xz.cn/extend/plugins/wordpress-seo/