Writing screenplays was a natural/logical 'extension of' his career as a novelist and his longtime interest in film.
He did some leg 'extensions'. [=exercises in which you extend your legs]
He disliked authority and, 'by extension', all government officials.
(..) Marge, haven't you finished that stupid letter yet? - " Homer sends his love. Happy holidays. - Marge! - The Simpsons." - Marge, where's the extension cord? For heaven's sake, Homer. It's in the utility drawer. Sorry. I'm just a big kid. And I love Christmas so much. (..)1989 TvShow: The Simpsons Title: Simpsons Roasting on an Open Fire Season: 1 Episode 1