It is now possible to upgrade your Arma 3 Standard Edition to the Arma 3 Digital Deluxe Edition.
To do this, please open your Steam Store (on the account where you already have the Standard Edition of Arma 3) and find Arma 3.
You should be able to buy Arma 3 - Digital Deluxe Upgrade.
Please note that this upgrade includes the Digital Soundtrack, Map, and Tactical Guide. It does not include Arma: Cold War Assault.