Harris ("How I Met Your Mother," Cabaret, Rent, Assassins) is set to portray Bryan Adam, a board member at McKinley high who seeks to cut arts programs at the school.
He also happens to be a character from glee-club leader Will Schuester's past. Flashbacks will show Harris' character getting all the attention and the girls, while Schuester (Morrison) sits off to the sides.
Joss Whedon will direct the episode, which promises an intense duet between the rivals.
"Glee" will resume with all-new episodes April 13 at 9 PM ET on FOX. The series has also been picked up for a second season. Spring Awakening's Lea Michele and Jenna Ushkowitz also star.