I would like to point all requests for a certain directory and all its possible subdirs to one documentroot.
So, for example, I get a lot of requests on many different subdirs here: domain.com/directory/and_all_possible_subdirs
Point to the index.php file that resides in domain.com/directory/
What to add in apache2.conf ?
I would use mod_rewrite for achieving your goal, that will be much easier. see http://www.phpriot.com/articles/search-engine-urls/5 for an example. You "misuse" the 404 condition that arises when entering a "wrong" URL an redirect the user to the wanted file/page.