I've yet to watch the last season...pretty much spoiler-free so I don't know why Supergirl has to join the cast. But they already have someone to play her.
Supergirl is coming to the CW's Smallville. Canadian actress Laura Vandervoort just got the call Wednesday that she has been cast and will start shooting with Tom Welling, Michael Rosenbaum and Aaron Ashmore next week in Vancouver. --- People Magazine
Supergirl's Kryptonian name is Kara.
Wasn't
Adrianne Palicki the original Kara in a previous season?
Note to self: must watch Smallville Season 6 pronto!
[Smallville], [Smallville's Supergirl], [Laura Vandervoort]
