The Scottish qualified back, who spent three weeks on trial at Scotstoun, will also play for Glasgow Hawks in the BT Premiership when he's not selected for the Warriors.
After being brought up in Christchurch in New Zealand, he has featured for Marist Albion Rugby Club and Canterbury age grade sides.
During that time he played against Warriors fly-half Finn Russell when he was over in New Zealand on the Macphail Scholarship in 2013.
Hagen qualifies for Scotland through his grandmother.