The former Wales captain, 33, joins Rovers for the 2021 season on a one-year deal.
Kopczak has previously played for Bradford, Huddersfield and Salford as well as Trinity and is waiting to see if his Super League career will continue, with Featherstone’s application to become the 12th team under consideration.
Rovers head coach James Webster said: “Craig brings us plenty of size and experience having played over 300 Super League games.
“Adding him has really strengthened competition for places in the middle of the pitch. With what we have seen this year in how coronavirus has affected the availability of players in Super League, having strength in depth in the middle positions was a vital area we needed to secure.”
Kopczak added: “I’m over the moon to be here and I’m looking forward to the challenge ahead.
“If the Super League application is successful, that will be great. If not, I can use my experience and leadership in the Championship to push for a Super League position through promotion in 2021.”
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