Onyango, 31, spent six years at Oldham, during which time the Roughyeds appeared in four Grand Finals.
He started his rugby league career at Widnes in 2006, having been spotted playing rugby union, which he played at the 2002 Commonwealth Games.
Head Coach Tony Benson said: “He is a very experienced winger, Lucas is also a qualified sports scientist and keeps himself very fit. He scores lots of tries, is very strong defensively and is good with the high ball.
“Lucas is a great character and will become a fans favourite.”