It's an interesting question. Technically addons are client side where the game is installed. Playing on stadia, you are streaming the game from a remote machine where the game is installed. Do you as a stadia user have file system access to that machine? Can you manage downloads and installations to it? Would seem a security risk if that were the case. I'd assume the answer is no for that reason, unless there is some kind of dedicated addon tool/manager.

I'm not a stadia user so I can't answer and my thoughts above could be totally wrong. I'd be interested in the answer though.