The veteran actor has confirmed he will play a love interest of Felicity Huffman's character's mum Stella Wingfield, played by Polly Bergen.
He tells The Hollywood Reporter, "I just decided (to do it), and am going to shoot next week."
But Hagman admits he is clueless about the hit programme - and is cramming in as many episodes as possible before he shoots his scenes: "I can watch a hundred episodes on my iPad. I never watch television, so I better get started so I can see what the show's all about."
Hagman's scenes will air next year (11).