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: