There are already a bunch of rules in my Apache configuration. What I want to add is the following. There are some shared folders (.shared): /var/www/.shared/ and /var/www/.include/.shared/ and /var/www/.include/(.*)/.shared/. Now when someone visits http://domain.com/test.png it first executes the existing apache rules and will (when the file/folder was not found) look in those .shared folders.
So suppose I've got this filesystem: /var/www/.shared/dog.png /var/www/.shared/test.gif and /var/www/domain.com/dog.png. Now when someone visits http://domain.com/test.gif, it must load the test.gif from the .shared folder. Now when someone visits http://domain.com/dog.png it must load the dog.png from the domain.com folder (because the existing apache rules will be executed first).