Forrest spent part of last season with the Seahawks. The team initially signed him to the practice squad in November before promoting him.
He appeared in two games before going on injured reserve.
Forrest originally entered the league as a sixth-round pick of the Rams in 2016. He played 10 games with the Rams, starting four.
Boykins signed with the Titans as an undrafted free agent out of UCF in 2017 and later spent time on Tennessee’s practice squad.
Carter, a third-round pick in the 2015 draft, had two stints on Seattle’s practice squad last season.
Hamm signed with Seattle last week before the start of camp.