Yes, you address the message to a public distribution list that contains all the user. by default the "All Subscribers" distribution list is created and all subscribers created on the box are added to it (unless you changed that behavior in the subscriber templates).
The administration guide covers the use of distribution lists - you'll find all the admin guides for all versions of Unity (including 2.4.6) here:
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_administration_guides_list.html
