The actresses will both appear in the forthcoming fourth season of the TV musical drama series, though details of their roles are being kept under wraps.
However, broadcaster Fox have confirmed the former 'Sex and the City' star will appear in at least one episode, while Kate's storyline arc will last for six episodes.
The network also vowed the show - which experienced a drop in ratings during the current season, which ends in the US on May 22 - is "poised for a creative renaissance" when it returns in a new Thursday night slot later in the year.
Fox's entertainment president Kevin Reilly explained the programme will become a "show within a show" following some of the McKinley High graduates as they attend a New York performing arts school, as well as introducing "some new faces" at the main Ohio educational establishment.
Last week, Amber Riley tweeted to apparently reveal she is leaving her role as Mercedes Jones when the character graduates at the end of this series.
However, she later retracted her comments, tweeting: "I was not confirming I was leaving the show. Just stating that I would miss being in the classroom everyday with the cast and that would be changing."
Existing stars, including Lea Michele, whose characters are also set to graduate are expected to remain with the show in season four.