Well, finally i found a solution myself :-)
The two plugins
https://www.dokuwiki.org/plugin:redirect and
https://www.dokuwiki.org/plugin:404manager are very promising. I've used the "redirect" with the "wildcard-patch" given on the plugin-page. It will redirect whole namespaces (with all subnamespaces) to a given page, or another namespace (if documentname is kept). Really nice and handy.
The other one, the 4040manager can also make suggestions for the user, to find the best match to his searched page-ID. Not always needed, but very good idea!