Kurt Morath signed for the Knights a couple of weeks before the beginning of the 2018/19 Championship season.
The Fly-Half is the highest point scorer in Tongan Rugby history.
The 33-year-old first action in a Doncaster shirt came in the dying embers of the opening match of the Championship season against Cornish Pirates.
Kurt's previous clubs include: San Diego breakers, Biarritz and Utah Warriors and will add a wealth of experience to the Knights' squad.
As of late January, the Tongan had managed 56 points from eight starts.