How do I add an app that isn't in Spanbox's default list?

It depends on one thing: does your app have a web version? Spanbox works by loading web apps, so the supported list is any app with a proper web login, and it's 60+ and growing. Check the list first (a lot is there beyond the headline names). If yours has a web app and isn't listed yet, it's a good candidate to request. Apps with no web version at all (Signal, Viber, LINE) can't be added until they ship one.