An external usb wifi adapter should generally work. The key being that the linux kernal has the required driver/module to support that usb driver. Generally any common wifi adapter will have linux kernel support. I would avoid trying to get the cheapest usb wifi adapter you can find, as those are more likely in my experience to not have linux support.
Making sure that I am providing a complete answer to your question, the lack of support for the internal wifi on TV boxes is unrelated to support for any specific usb wifi adapter.
