Masella, a prop forward who has played for Illawarra, Balmain and South Sydney, represented Tonga in last year's World Cup. He was originally recommended to Leeds by Mike McClennan, the former St Helens coach who was in charge of the Tongan national side.
Collins, a former St George and South Queensland Crushers hooker, was a member of the Canberra Raiders squad beaten 30-18 by Widnes in the World Club Challenge at Old Trafford in 1989.
Tony Fisher, who guided Doncaster to the top flight two years ago, has been appointed as coach by the Second Division club for the second time. He replaces the Australian Peter Regan, who moved to Batley in October.