St Helens must free an overseas-player spot for Tom Starling, whose arrival would take the club to 11 overseas players against Super League's limit of 10.
The club confirmed the Canberra hooker has signed a three-year deal beginning next season.
Starling, 28, was contracted to the Raiders until the end of 2027, and November 1 was the date he could first negotiate with rival clubs.
Canberra instead gave him permission to seek a move a year early, with more playing time understood to be a key motivation.
He had fallen behind Owen Pattie in coach Ricky Stuart's hooker selection order.
His new club's other NRL recruits include Sean O'Sullivan, Christian Tuipulotu and Solomona Faataape.
Tristan Sailor, Daniel Suluka-Fifita, Curtis Sironen, Shane Wright, Nene Macdonald, David Klemmer and Jacob Host could be affected by the quota issue.
Love Rugby League understands St Helens has offered Wright to rival clubs for next season, while Macdonald is also considered likely to leave.
Starling said working with coach Kevin Walters was a major attraction: "This is such a great opportunity to be able to join such a prestigious club as St Helens," he told the media.