Hubbard, 26, reportedly agreed to a five-year pact worth $37.5 million, Ian Rapoport of the NFL Network reported. Stephenson's deal is for one year and $2.5 million.
Deals cannot be officially signed until Wednesday afternoon.
The 6-foot-4, 286-pound Hubbard appeared in all 16 games, making a career-high 10 starts for the Steelers last season, seeing time at both tackle and guard. He started four of 15 games in 2016.
Stephenson, 29, spent the past two seasons with Denver, starting a career-high 12 games in 2016. He appeared in all 16 games in three of his first four seasons with the Kansas City Chiefs, making 21 starts.
Thomas still has not told the team whether he plans to return in 2018. Last season, Thomas suffered a season-ending torn triceps injury on Oct. 22 and failed to play 16 games for the first time in his career.
Named to the All-Pro first team six times, Thomas has said he is not worried about the triceps tendon as much as nagging injuries to both his knee and back.