Adding your Blogger-based blog to Google Webmaster Tools
First of all, in the Dashboard, add your blogspot.com hosted blog by typing your blog’s URL in the text box and pressing the “Add site” button. Now you have to verify it; just follow this simple how-to: “Add Google Webmaster Tools to your Blogger Blog - TakeOnTheNet”.
Adding the Sitemap(s)
Now the last step: we have to add the complete sitemap. We’ll use the blog’s feed as a Sitemap, but to get the complete feed we have to use some addictional parameters.
- Log into Google Webmaster Tools, enter your blog’s properties and select e Add sitemap;
- select “Add general web sitemap“;
- my sitemap url is: atom.xml?max-results=500&start-index=1
- press “Add general web sitemap“.
If you need more sitemaps, repeat the four steps, changing only the url; just add 500 to the last number of the previous sitemap’s url:
- atom.xml?max-results=500&start-index=501
- atom.xml?max-results=500&start-index=1001
- atom.xml?max-results=500&start-index=1501
- …
and so on.
Important: in Sitemap > Overview, the “URLs submitted” column will show effective number of articles submitted +1 (so, a full sitemap will show 501 urls, not 500), because, in the feed, the URL of the home page is also retrieved.
Feed’s parameters explanation
- max-results: the max number of articles to put in the feed - a sitemap with too many articles could exceed its limits (50000 articles or a sitemap file size of 10 MB - remember: the feed doesn’t retrieve only links, but also articles’ bodies); we’ll use this parameter to split the sitemap in sub-sitemaps;
- start-index: the index of the first article to be retrieved in the sub-sitemap: if max-results is 500, then this index is 1 for the first sitemap, 501 for the second (if we have more than 500 articles), 1001 for the third, and so on.
So the sitemap’s url looks like this:
http://yourblog.blogspot.com/atom.xml?max-results=500&start-index=((N-1)*500+1)
with N the Nth sub-sitemap you’re asking for.
To put it simple, just add +500 at the last number of the url, every time you need a new sub-sitemap. 
(for more info, read the Google Data APIs Protocol Reference)
PS: I’ve discovered that another one found a similar solution (next time I’ll search better before reading Google APIs 
): http://phydeaux3.blogspot.com/2007/10/blogger-feedburner-sitemaps-and.html
He suggests to use a smaller number for max-results; so, if you experience problems with 500, just use 100.