Is iMessage supported in Spanbox yet?

Not currently, and honestly not soon. iMessage has no web version at all and is locked to Apple's own devices, so there's nothing for Spanbox to load (it works by running each app's web app locally). Unlike Signal or LINE, there isn't even a web client on the horizon from Apple. You'd keep iMessage on your Mac or iPhone and use Spanbox for everything that does have a web app.