The Terps open their season Saturday with a road matchup against Northwestern.
The three-star product joins the Terps from nearby St. John's College High School.
Castleberry didn't receive a carry for Michigan last season.
Harrison suffered a season-ending knee injury against Penn State on Sept. 27.
Coach Mike Locksley has focused on building the program's foundation in his first year.
Leake and Anthony McFarland combined for 66 percent of the team's rushing yards in 2019.
While Locksley and the Terps are out of bowl contention, they're confident in the future of the program.
“We’ve still got a lot more to prove in the two games left.”
In the Terps' past five losses, their opponents have scored on at least their first two attempts.
The Buckeyes took a 42-0 lead into halftime.