The 2003 World Cup winner assisted Robbie Deans last season when the Barbarians drew 31-31 with South Africa at Wembley before wins over the Czech Republic (71-0) and Fiji (40-7).
He will also be working alongside Scott Robertson, the Crusaders head coach who claimed the Super Rugby title this summer.
“I’m really looking forward to doing it and it was a great experience working with Robbie last time,” said Greenwood.
“I watched Scott and the Crusaders win Super Rugby, his style is hugely exciting and it’s a great opportunity for me to keep learning.”
Greenwood faced New Zealand six times as a player in his distinguished career – four times with England and twice with the British & Irish Lions.
He watched the All Blacks “up close” this summer in the drawn Lions series and the Barbarians will have to be at their very best to emulate their famous victories in 1973 and 2009.