As the PP said, some people have reported that they have been able to combine their parkhopper and vouchers to upgrade, I read one post that said they were not allowed. According to the website you cannot combine the two:
May I upgrade my free one day ticket to an Annual Pass or multi-day ticket?
Yes, you may upgrade the free ticket to another ticket type of greater value within the applicable upgrade window. However, you are responsible for the incremental cost and the new ticket may only be used by you. No changes or refunds will be given. The value of the free ticket may not be used to combine or upgrade with a separate single or multi-day ticket.
We plan on using our voucher to upgrade to a parkhopper, then after we use the MM we will upgrade the parkhopper to the AP.