Created by Tina Fey and Robert Carlock, the sitcom ran for four seasons and starred Ellie Kemper, Tituss Burgess, Jane Krakowski and Carol Kane -- all of whom have returned for the standalone installment.
The followup special is set to air in 2020.
"In the Unbreakable Kimmy Schmidt interactive special, Kimmy Schmidt (Kemper) sets off on her biggest adventure yet. Three states! Explosions! A dancing hamburger! And you, the viewer, get to decide how the story goes," a press release said. "Will you foil the Reverend's (Jon Hamm) evil plan and get Kimmy to her wedding on time? Or will you accidentally start a war against the robots?"
A photo posted on the Netflix Twitter feed said Radcliffe will be playing a character named Frederick.
No other details were immediately revealed.