We already know that Superman is a part of the CBS Supergirl universe as footage has featured the Man of Steel, albeit in more of a blurry form.
Speaking of The Blur, the actor who played Clark Kent and Superman on Smallville for ten seasons, Tom Welling, comments on potentially reprising the character for the new Supergirl series.
"There's people who would like it. There's people who don't like it," Welling told Buzz Feed. "But I will tell you that no one has ever asked me to do it. So, I don't know. It would be kind of odd. I don't know who he is at that point. He would obviously be Superman..."
As seen in the final episode of Smallville, Welling would don the familar red and blue cape and costume and fly off as Superman. Perhaps Welling, now 38 years old, if not as Superman, could guest star as a different character. The Supergirl show already featured former Lois and Clark Superman, Dean Cain, as well as former Supergirl, Helen Slater, from the 1984 film.
Check out a scene featuring Superman saving Supergirl below as well as a Tom Welling Superman tribute video.