The prop joined North Queensland Cowboys on a train and replacement contract for the 2023 season but is still waiting for his first-grade debut.
Namo has played 39 games at Queensland Cup level since his debut for PNG Hunters in 2021.
The 23-year-old will link up with fellow new signing and PNG team-mate Nixon Putt at Wheldon Road, as well as Lebanon international Elie El-Zakhem.
“I can’t wait to make the most out of it and give my best to the club," said Namo, who represented his country three times at last year's World Cup.
"I’m a little bit nervous to be travelling alone compared to the World Cup when I came with a full squad but it’s going to be a great moment for me going overseas to play in England, especially playing at Castleford Tigers.
“We have a few PNG boys who will be good together. Liam Horne was telling me that the fans here are crazy and I can’t wait to come and meet them.
"I’ll be going onto the field to run hard, tackle hard and do my job”.