Is Viber supported in Spanbox yet?

Not yet, and the honest reason is Viber has no official web app, so there's nothing for Spanbox to run locally (Spanbox loads each app's web version on your machine). It's on the roadmap for if and when Viber ships a web client, it's in the same boat as Signal, Viber, LINE and KakaoTalk. For now you'd keep using the Viber app itself.