I chattet with the Sony Online Support last night (well at least it was night in Germany
).
The solution was so simple I wonder why I didn't see it.
In the reader software go to the preferences dialog and select "USA" in the 'territory/region' setting
This way the software does not redirect to Germany ebook stores like thalia.de and I can log in using my existing sony account.
When I re-downloaded my previous purchases I got them in .epub already - but of course with the included DRM. Converting to a Kindle-compatible .mobi using Calibre and a plugin worked fine.