I am creating an app with a Pro-feature which can be unlocked via in-app purchasing.
Is it possible to use the Windows Phone 8.1 Wallet API for creating a Voucher? (As example for Reviewers or tester)
How can I ensure that the voucher can be used only by one person?
(maybe: can only be used for a particular LiveID)
There isn't currently any voucher or promo-code system available for Windows Phone apps.
You could implement this yourself along with using the existing in-app purchase system. Check the code yourself and then enable the feature if either the code is present or if the in-app purchase was made.
Tying it to a login such as the LiveID would be a good way to lock it down to one person, especially if your app already has a login requirement. Another option would be to make the code one-use and then roam the unlocked status once it was set. That would let the first user to use the code use it on multiple devices so long as the app wasn't fully uninstalled or was used often enough to keep the roamed data alive.
Prepaid codes are frequent request on the Windows Platform Dev Feedback site. You can add your vote at http://wpdev.uservoice.com/forums/110705-dev-platform/suggestions/2118985-prepaid-codes-for-wp-apps-promo-codes-for-review