Using the GUI, I could not find a way to move it around. I already thought about manually changing the configuration, editing all the references for all accounts.
Fortunately, when looking around the
~/.claws-maildirectory that contains everything, I found
folderlist.xmlwhich is an easy to understand (and edit) XML file that describes how the folders are displayed. I did no fancy stuff, just moved the <folder type="imap"...> ... </folder> block, that corresponds to the new account up to the first place and voilá -- it works.
BTW: Claws-Mail is still my favourite email client after using it for over a year - that clearly is a new record ;)