The Tigers have also agreed a deal with Hlad's compatriot Ned Catic, subject to the 29-year-old prop being granted a work permit.
The club said the Australian duo had signed for the rest of the season.
The 21-year-old Hlad is a British passport holder and can play in the front or back-row.
He was a member of the Panthers' Jersey Flegg grand final-winning side last year and is keen to make an impact in first grade.
"This is a big opportunity for me and a great chance to help the Tigers," Hlad said.
"I have a very good work rate, I'm very committed and can give plenty of game time."
Castleford football manager Michael Robinson agreed Hlad was a hard worker.
"There is a lot of competition at the Panthers and that has enabled us to secure his release," Robinson said.
"We feel that given this opportunity he will do a great job for us."
Robinson hopes to have Catic available to play against his former Super League club, Wakefield, on May 3.
Catic also played at the Panthers before joining the Sydney Roosters, where he was a grand finalist in 2003 and 2004.
After three years with Sydney, he moved to the UK in 2006 and had two Super League terms with the Wildcats.
He has just completed the French season with Lyon.
Catic said: "I know that there are only four months of the season left but I am looking forward to making an impact."