No, you can't combine two tickets to make one upgraded ticket. What you can do is downgrade your package to one day tickets (if you need to have a package to get dining, for example), redeem your vouchers and upgrade them to the tickets you need, and save the package tickets for another trip. The tickets from your package will not expire until you use them, so they can be saved indefinitely and will give you a head start on the next trip.
If you are within 45 days, Disney may charge a $50 change fee to downgrade your package.
Alternatively, you can redeem your GAD voucher for a one-day ticket, then use one of your package days as a hopper for that day only. You generally can't use two days of park tickets in the same day, but since you will have two physically separate tickets per person (the package ticket and the GAD one-day ticket), you could effectively use the GAD voucher as a hopper for one day.
The cost difference between a 3-day base ticket and a 4-day base ticket is only about $6 per ticket, so using the GAD voucher as your fourth day doesn't assign it a lot of value. If you only need three days and want to keep your package as is, you can redeem the voucher for Fastpass cards for your group or for the figurine and pins, or you can donate it.
