Yes, you should be able to. You can upgrade the GAD tickets to Florida resident tickets or other multi-day tickets, so there's no reason why you can't. From Cheshire Figment's "Everything About WDW Tickets" sticky on the Theme Parks board, regarding the military tickets:
"If you have unused non-promotional tickets and wish to have them applied to the purchase of Tickets, this may be done provided the value of the unused tickets is equal to or less than the value of the tickets being purchased or upgraded."
BUT, you will have to do it at a Disney ticketing window, not at Shades of Green or at your local MWR/ITT office, and you will have to pay sales tax on the purchase.
There's a huge thread on the Theme Parks board about the Military Salute 2010, if you search for that thread, I'm sure you'll get a definitive answer from someone who's done it. But, since the GAD tickets are upgrade-able, there's no reason why you wouldn't be allowed to upgrade to the Military Salute tickets.