Greenwood spent most of last season on a dual registration with South Wales Scorpions, although he did make his Super League debut for the Warriors last August.
Wigan coach Shaun Wane said: "This is a great opportunity for James to gain some experience at the highest level.
"It will give him the chance to play regularly in Super League with London and it will also offer him some stability off the field knowing he is with the Broncos for the whole year."
Greenwood joins Hull hooker James Cunningham and the Leeds trio of James Duckworth, Thomas Minns and Alex Foster at Barnet, along with permanent signings George Griffin (Hull KR), Denny Solomona (Melbourne), Scott Moore (North Queensland) and Nesiasi Mataitonga (Cronulla).
Broncos coach Tony Rea said: "We are excited to welcome James to the capital. He is a powerful player who has developed a real presence on the field. It is a terrific move for all concerned."
- Sports & Recreation
- South Wales Scorpions
- dual registration
- James Duckworth