Caqusau has played age grade rugby for Scotland but is also qualified to represent England.
He's played club rugby in Scotland with GHA and Ayr.
"We first noticed Amena playing in the Under-20s Six Nations last year, while we were watching Henry Pollock, Craig Wright and Archie McParland, and his natural athleticism was clear to see," said Saints boss Phil Dowson.
"His father hails from Fiji and played Sevens and XVs for the British Army, and Amena has that very Fijian ability to beat players, to be powerful in contact, and to get offloads away.
"We really liked the way he played, and then we did our research and he came across as a really hardworking character, who wasn't necessarily playing much up in Glasgow. We felt that within our pathway system, he had huge potential."