I don't know of a way to get apps without wifi.
The problem is that you need to register your nook with BN (which you can do from anywhere, granted) but then to shop in the store and buy apps, they need to deliver them straight to your nook.
Getting apps (even for straight tablets) "sideloaded," instead of installed from the manufacturer's site, is hard. Not a nook-specific thing.
You could download the apps while you are at a local hotspot, though, and keep the device offline most of the time.