It should be possible, the Moodle DOCS say this:
- If an alias is created, then when the original file is updated, it will change in all instances of the alias. So a teacher might add course notes to their private files for example, and then add them into a number of courses. When the teacher makes an alteration in the file in their private files, this alteration will be reflected in all instances of the course notes.
http://docs.moodle.org/23/en/Working_with_files
But... how?