The 34-year-old All Blacks centre will play in Super League for the first time in 2020 after signing a two-year deal with newly-promoted Toronto Wolfpack.
"I think it's great for the sport," Marshall said on the eve of the Kiwis' second Test against Great Britain in Christchurch. "Obviously Sonny Bill is a great athlete and coming back to rugby league is a real coup.
"I think he's been a great role model for both codes, especially young kids growing up in New Zealand who probably don't feel they have the opportunity other kids do.
"He's shown that with hard work and determination, you can do whatever you want."
Williams played for the Kiwis in the 2013 Rugby League World Cup final but both Marshall and coach Michael Maguire were tight-lipped on his potential return to the fold amid a suggestion that he is set to switch his allegiance to Samoa, where his father John was born.
Maguire said: "In time that will sort itself out."
Meanwhile, Maguire says forward Kenny Bromwich will not play in the second Test on Saturday after staying in Melbourne to be with his pregnant wife.
Maguire had left a spot open for Bromwich, whose last-ditch tackle on winger Jermaine McGillvary in Auckland last Saturday enabled his side to hang on for a 12-8 win, but said after the captain's run that his place on the bench will go to Kevin Proctor.
"He's done the right thing, he's going to be there with his wife for the baby." Maguire said.
"Kevin Proctor has got a lot of experience and he's a senior player around the group so I think we've got a good replacement."
Maguire has recalled half-back Shaun Johnson in place of the injured Kieran Foran and handed another chance to hooker Brandon Smith after dropping him a week ago for "failing to meet standards".
"Brandon is aware now of what we expect," he said.
Great Britain coach Wayne Bennett has been forced into making further changes due to injury and has chosen specialist stand-off Blake Austin on the wing but Marshall will take little encouragement from the Lions' woes.
"Any team coached by Wayne Bennett are dangerous," Marshall said. "Whoever comes in does their job and I'm sure Wayne will have them prepared to do that."
