There isn't an addon for it, because the alex voice was created by apple, I believe, and thus is copyrighted. Now, what you can do is download something called Macintalk, which acts as a bridge between your mac, if you have one (And I think it works on virtual machines of OS 10 as well), and the NVDA synthesiser. This voice bridge will allow you to use Alex just as any other voice. You will need to be connected to a network if you want to get it to work.. Unfortunately, you can't bridge with your phone.