The Tonga international, highly-regarded in the NRL, will join the West Yorkshire club on a three-year deal from 2020.
It has completed a busy week of announcements for Castleford, who host Leeds Rhinos tomorrow night having also confirmed deals for Salford Red Devils wideman Derrell Olpherts and prop George Griffin in recent days.
Plans for 2020 are shaping up and Powell could switch Cheyse Blair – the Australian centre signed from Melbourne Storm in April – from left centre to right next season once captain Michael Shenton has recovered from injury.
Asked if Feki would then partner Cheyse on the right, where Tigers have had a number of issues this term, the coach said: “We’ve got competition for places. We’ve signed Derrell Olpherts who is a winger who can play other positions as well.
“There’s certainly going to be competition for places next year in our outside backs which has probably been a bit of a criticism of us from our own fans – that we haven’t been able to recruit heavily in the outside backs.
“We certainly do have that now over this last period so we’ll see where this takes us next season.
“We’re still in this one – and want to be successful in this season – but moving forward we look strong in all areas.
“We just need to have our better players on the field at the moment which has been tough this season.”
A strong ball-carrier and excellent finisher, Feki has scored 62 tries in 139 games for Cronulla since debuting in 2013.
Importantly, the Aucklander knows what it takes to lift a title, too, having been part of the Sharks side that won the 2016 Grand Final.
Powell added: “Sosaia is a player who has been performing at the top level for a number of years in the NRL.
“He was a part of the Cronulla team that won the title in Australia which means that our last two recruits from overseas know what it takes to win a championship.
“Experience-wise that is always invaluable.
“He is big, strong and aggressive with the ball and an experienced defender.
“He can make the game really quick for you and is an international as well so he’s a great signing for us. Our back line for next season has a real look of ability and Sosaia adds hugely to our quality.”
Feki, 28, said: “I’ve been at Cronulla for seven years now.
“I’ve been comfortable here but this is a new chapter in our lives and my partner and I are really excited to come across to Castleford and to Super League.
“It’s going to be a new challenge but Cronulla play an attacking style of rugby as well so hopefully I can bring that over.
“I’ve played with Junior Moors at the Sharks and I know Jesse Sene-Lefao already as well.
“I’ve been messaging him and he’s been tipping me up on the club and what it’s like.”
He added: “As a player, I just try to do my job for the team and finish any opportunities that come my way.
“I’ve played a lot of my games with the Sharks on the left but I’m also really comfortable on the right and I played on that side at the weekend.
“I’m not bothered where the coaches want me to play, wherever they put me I’ll come across and do my best for the club.”
Castleford’s current right winger Tuoyo Egodo is ruled out due to concussion tomorrow – as is half-back Jake Trueman who misses his first game of the season.
Left winger Greg Eden and Mitch Clark come back in while Leeds could hand a debut to new signing Rhyse Martin who arrived yesterday from Canterbury Bulldogs.
Hull KR make just one change to their squad for tomorrow’s game at Wigan Warriors, Jimmy Keinhorst replacing Tommy Lee.
Hull FC welcome back Gareth Ellis and Albert Kelly for the visit of leaders St Helens but Wakefield Trinity have lost Matty Ashurst for the rest of the season due to a back injury.