The movie, titled "The Town That Came A-Courtin', is based on a novel by Ronda Rich and also stars Lauren Holly, Cameron Bancroft and Lucie Guest.
UP, which described the movie as an uplifting romance, said Wednesday it welcomed the "inspirational" Harper as part of the project.
In March, the 73-year-old actress said she's been diagnosed with a rare, incurable brain cancer but that she intended to live each moment fully.
Harper gained fame playing Rhoda Morgenstern on television's "The Mary Tyler Moore Show" and "Rhoda."
UP's "The Town That Came A-Courtin'" will air in January 2014, the channel said. It's being shot in Vancouver, British Columbia.