Both players have joined up with the Warriors squad for pre-season training.
Ratu has made two appearances and has scored one try since joining in November 2016, will play for Currie Chieftains in the BT Premiership when not involved with Glasgow next season.
The Australian-born former NRL player will be staying at Scotstoun until at least May 2018.
The 26-year-old told Warriors TV: "I'm excited to be staying on and continuing to learn under the new coaching staff.
"Dave Rennie's coaching record speaks for itself, so it's phenomenal to be able to learn from someone like that.
"I'm really enjoying it here and I'm grateful to have the chance to stay next season."
Glasgow-born Kiran has pulled on a Warriors jersey before, featuring in the 63-0 victory over Canada 'A' in pre-season last year.
The 22-year-old has also played for Glasgow Hawks in the BT Premiership.
The former BT Sport Scottish Rugby Academy member has spent a year in England playing for Hull and returns to Glasgow to bolster the club's strength in depth in the second-row.
Standing at 6ft 8in, Kiran will join Brian Alainu'uese and Greg Peterson as the tallest members of the squad next season.
Speaking to Warriors TV he said: "It's an honour to come back here. I've never stopped following Glasgow - it's my home, and I've never stopped pushing to try and get back up the road.
"Jonathan Humphreys has done really well with Scotland. I've been lucky enough to train under him a couple of times before and he's a great coach.
"Dave Rennie's coaching experience is unbelievable and he's achieved so much at the Chiefs, so it's very exciting."
Glasgow Warriors assistant coach, Kenny Murray added: "Ratu impressed playing for Currie and in his opportunities in a Glasgow shirt last season and has earned another year at the club.
"We expect him to continue to progress as he harnesses his natural athleticism and develops his knowledge of the game and the way we want to play.
"Kiran has spent time with us before and it's great to welcome him back to Scotstoun.
"He's an imposing and mobile player and will add to the competition we have in the second-row next season."