Gifting a Digital Upgrade

You can gift the original-vanilla-World of Warcraft digital product, Keriam, however, as stated above, you are not going to be able to gift a digital copy of Wrath of the Lich King. You may purchase the retail copy and send that off as a gift, if you would like, though, as there are no rules restricting you from doing so.

Alternatively, have your friend digitally upgrade their own account via the Blizzard Store. You could try using a gifted pre-paid credit card for the end result that you desire. While I am unable to guarantee the usability, we do have many players that use pre-paid credit cards on various purchases-be it from the online store, of one of our several paid services.