Debbie starred as Grace's mother Bobbi Adler in the original series, which was rebooted with huge success last year. She died on December 28, 2016 - just one day after her daughter Carrie Fisher, Billie's mother, who passed away following a heart attack.
Billie will shoot the episode on September 25, with the plot showing her character Fiona reconnecting with her "cool aunt" Grace, according to Deadline.
"The episode is especially meaningful because of Billie's relationship to the show," "Will & Grace" co-creator and executive producer Max Mutchnick said. "We're thrilled to have Billie. And best of all – she's really good."