Yes, that seems to be the most obvious answer, and maybe too simple for me to think about it. :)
But there is one small problem: when the wiki was created, I did not set up separated namespaces, because it absolutely wasn't necessary at that time. So the path (structure) of the wiki looks like this:
/somepath/dokuwiki/pages/
/somepath/dokuwiki/attic/
/somepath/dokuwiki/cache/
/somepath/dokuwiki/deleted.files
/somepath/dokuwiki/index/
/somepath/dokuwiki/locks/
/somepath/dokuwiki/media/
/somepath/dokuwiki/media_attic/
/somepath/dokuwiki/media_meta/
/somepath/dokuwiki/meta/
/somepath/dokuwiki/pages/
/somepath/dokuwiki/tmp/
/somepath/dokuwiki/tpl/
/somepath/dokuwiki/
"somepath" is outside the installation path to ease the backup procedure. All pages are located in the directory "pages", while all the media is located in the directory "media". This may not be the most intelligent places for the content to be put into, but, as I said, the wiki started with different conditions.
Can I realize your solution now without breaking the wiki? I know I can simply create a new namespace by creating a new page in it, eg. "newnamespace:newpage". I've tried this, and that will create a new namespace, but the according directory is being created as
/somepath/dokuwiki/pages/newnamespace/newpage.txt
I am not sure, if this is meant to be that way. I would have expected the new page to exist in
/somepath/dokuwiki/newnamespace/pages/newpage.txt
And uploaded pictures should then go to
/somepath/dokuwiki/newnamespace/media/picture.jpg
Most probably I've made a fundamental mistake when I set up the wiki. I would be very thankful if you (or any one else) could give me a little help here.