Curse of Strahd is a module, not an addon. Addons are smaller adventures, which can be added to most campaigns without issue. Modules are much larger and contain enough assets and resources that they would likely perform very slowly or problematically if added to an existing game. Modules are meant to be used during game creation. Since you are Pro, your best bet would be to create a new game and choose Curse of Strahd at creation time, then transmogrify in any assets, characters, playlists, etc. after you have created it.