I'm not actually sure that workaround works. I've opened the Steam overlay while in Neverwinter and there's no option for buying Zen w/ your Steam Wallet that I can see. In fact, I don't see anything related to payment at all in the overlay. This combined w/ the fact that all the Neverwinter DLC was removed a month ago makes it feel like something is going on.
Comments
http://www.arcgames.com/en/forums/neverwinter/#/discussion/1222526/steam-wallet-payment-removed there's a workaround posted in the thread here so it doesn't seem to be intentional
Here's the link for the DLC removal announcement: http://www.arcgames.com/en/forums/neverwinter#/discussion/1220745