The group joins previously announced cast members Naomi Watts, Josh Whitehouse, Georgie Henley, Naomi Ackie, Denise Gough, Jamie Campbell Bower, Sheila Atim, Ivanno Jeremiah, Alex Sharp and Toby Regbo.
Goldman is writing the prequel based on a story by her and Martin.
The series will take place thousands of years before the events of HBO's "Game of Thrones." SJ Clarkson is attached to direct the pilot episode.
"Game of Thrones" will be coming to an end with the premiere of the drama's eighth and final season on April 14.
The show is based on Martin's unfinished "A Song of Ice and Fire" book series.
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