"Now it's official," Nick told Extra. "I still don't know what she's doing, but she's doing something! She's ready... she's excited!"
As Nick says, Mariah's role on the show has yet to be determined. It's unlikely the show would add another judge this late, so Mariah will either be a guest on the show or help mentor the contestants. The term being thrown around is "guest mentor" and "judge of the judges" for her role.
The X Factor will debut on September 21.