The answer is no, you can't. As StitchandPooh'sMom already pointed out, with very few special exceptions you cannot combine two tickets into one, which is what you are asking.
Your GAD voucher itself has no dollar value. You can use it to get a GAD ticket, or if you have a multi-day ticket or AP already you can get the other options.
Now, the GAD ticket itself has value, but it is a ticket, and therefore you are running into the "cannot combine tickets" rule.
You could apply the GAD ticket value to a renewal of an AP, if you are at least within your renewal window. When you renew, you can up/downgrade your AP at the appropriate renewal price.