The dual code international, who confirmed his retirement this week, will provide opinion and analysis during Australia's National Rugby League (NRL) and Super Rugby AU competitions along with New Zealand's Super Rugby Aotearoa tournament.
Williams, whose father is Samoan, said he looked forward to a new challenge and hoped to inspire young Polynesian players by taking on the role.
"I know I represent a lot of young Polynesian boys, Kiwi boys who represent so much on the field and when it comes to off the field we're not really in this space," he told Stan in an interview.
"So just being able to jump into that space and see how we go, is what I'm looking forward to the most."
Williams's appointment comes weeks after former Australia coach Michael Cheika and Wallabies icon David Campese joined the Nine and Stan commentary team.
Williams, who won two rugby union World Cups for New Zealand in 2011 and 2015, played his final game of professional rugby league for the Sydney Roosters in last year's NRL semi-finals.
He is also keen to revive his boxing career after building a gym in Sydney, having dabbled in the sport some years ago.