The GAD ticket is treated just like a regular 1 day base MYW ticket and can be upgraded. Take your GAD voucher to a Will Call window at any of the 4 parks to get your GAD ticket - use it to enter the park. At some point within 14 days (same day if you like) you can go to any Guest Relations and ask them to upgrade it to the FL pass.
Make sure you bring valid ID with you - you'll need it for the GAD voucher and FL pass purchase.