With premiership-winning rake Api Koroisau joining the Tigers next season, Liddle was released from the final year of his contract to join the Dragons on a one-year deal.
But he is no guarantee to walk into the hooker role at his new club, with former State of Origin player McCullough rubbishing speculation he could consider retiring at the end of the season.
"I'm going to play on," 32-year-old McCullough told AAP before Liddle's signing was announced.
"I like competing, I like going to training.
"I might not be doing all the things I used to but I can contribute in other ways on the field. You learn that as you get older."
McCullough is contracted through to the end of 2023 but said speculation about his future had been warranted.
"That comes into it when you get to my age," he said.
"I wasn't happy with my form (this season) so I've got a lot to work on."
Liddle's move to the Dragons also comes following calls for Ben Hunt to shift to hooker, with Moses Mbye and Jayden Sullivan spending time there this season as well.
"Hooker is a position we identified as needing further depth in and Jacob is a good prospect for the club given his age and experience," Dragons general manager of football Ben Haran said.
Despite requesting an early release, 25-year-old Liddle leaves the Tigers on good terms after 75 first-grade games.
"He is a tremendous talent, a lovely young man, and an outstanding person," CEO Justin Pascoe said.
"We wish Jacob the very best for his future."
