Well the complicated answer was in Blogger.com's Help .
So it's possible but 10 steps are too complicated for me and sound risky.
The more general approach ist to use Blogging tools.
Unfourtunately w.bloggar doesn't offer a backup function so i have to save post by post. Not very practical in the future.
And there's the raw kind. Just fetch the HTML with a webspider like httptrack.
Any suggestions ?