I don’t recall that Extras were ever added later, but I suppose it is possible, as the supplying studio can upload an updated version. Usually new movies are released with bonus content from day 1 on this streaming platform, if available with the product at all.
Note that iTunes Extra can be available on Apple devices or in the iTunes application on Windows, not on third party devices with the Apple TV app. If you can't view your iTunes Extras - Apple Support
SD versions of movies do not have iTunes Extras, but HD and/or 4K versions are the same store item, with the same perks.
Rented movies do not get the iTunes Extras.