He joined Sheffield in December from Barnsley, who have also been promoted to Yorkshire 2.
New Zealand-born Posa is a second generation Croat and head coach of the Croatian national side.
Earlier this month they claimed a historic promotion to the second tier of European rugby for the first time in their history.
To top it all off, the University of Sheffield's director of rugby oversaw a varsity win against Sheffield Hallam and a second-placed finish in their BUCS league.
“It’s been a bit of a dream,” he said.
"I have been around long enough to appreciate you have to appreciate moments like this.”
The Croatian national side is normally bolstered by Australia and New Zealand-based players with Croatian heritage, but coronavirus restrictions meant Posa had to make do without his best talent.
He added: “We have done it with just local boys, which is huge.
"It’s not even semi-professional, they lose money when they play. The sacrifices they had to make, it’s phenomenal.
"I couldn’t be more proud of the guys. The Kiwi and Australian guys are normally our best players. To not even have them and achieve it...wow.”
Sheffield last competed in National League 2 North, the regionalised fourth tier of English rugby, during the 2017/18 season.
Doncaster-based Posa said: “Sheffield gave me a chance to bounce back, I’m chuffed for the club. There’s a lot of good people there.
“The obvious target is to survive. I think if we think with that attitude it’s not the way forward. For me, you go into it and try to win every game.
"When you have got guys enjoying their rugby and wanting to be part of it, it’s half the battle.”
Another reunion will be on the cards next season when Posa faces his old club, Rotherham Titans.
"It’s not about me, as much as it probably could be,” he said.
"They are still run like a Championship club, it will be David vs Goliath but we have got nothing to lose.”