Well, Answer to the Question is, Blogger wont allow to upload external sitemaps or whatever it is. But use this dynamic sitemap with webmasters yourname.blogspot.com/rss.xml <<<< This would do awesome work... i guarantee because i was just a blogspot blogger before 2 years
i already tried to do this but of no success, there's no way you can add your generated sitemap to a blogger blog. however i found this information in google support. Default blogger name http://blogname.blogspot.com/atom.xml?redirect=false&start-index=1&max-results=500 Custom domain name http://blogname.com/atom.xml?redirect=false&start-index=1&max-results=500 Code (markup): Note that the 500 is equivalent to the maximum no. of articles or links you have in your site. So far when I added this sitemap to google and bing webmaster they got the correct URLs submitted which is also the same to my generated sitemap via rage sitemap automator.
The Blogger sitemap as already said is [yourname].blogspot.com/sitemap.xml - you don't need to build one, this contains all your posts. Just add it to Google Webmaster Tools. If you need to verify your Blogger blog on Webmaster Tools, it does say to use Meta Tag verification, however if you have your Goodle Analytics code on Blogger, verify using that - it's automatic because Webmaster Tools already should know your Analytics account.