While I don't work for WIS, I am a skilled IT analyst... And my guess is that they would have to re-do their database structure to provide that. They use player-ID as an unique identifier, and having that would create a duplicate key situation in the DB. (Yes, I know partial years, combined seasons, etc... it might be possible to create a new type like that).
All in all, it would be significant work. And not used all that much on the site. It's one of those low-benefit/high cost changes.