Riddick's deal is for one year, a source told ESPN's Rob Demovsky.
Riddick, a 29-year-old who spent his first six NFL seasons with the Lions, hasn't played since the 2018 season. The Denver Broncos signed him in August 2019 after he was released by Detroit, but Riddick hurt his shoulder in a preseason game and was placed on injured reserve.
The Raiders already have a crowded running back room with starter Josh Jacobs, fifth-year veteran Jalen Richard, third-round draft pick Lynn Bowden Jr., returner Rod Smith, free-agent signees Devontae Booker and William Stanback and second-year fullback Alec Ingold.
While Smith and Booker spent time on the COVID-19 list earlier in camp, the Raiders also brought in running back Jeremy Hill on Aug. 3 before waiving him two days later.
Riddick has rushed for 1,023 yards in his career and has 2,238 receiving yards.
Emanuel, 29, started 32 of the 63 games he played in for the Chargers from 2015 through 2018 but retired last year, citing injuries.
The Raiders rebuilt their linebacker corps this offseason, signing free agents Cory Littleton and Nick Kwiatkoski, drafting Tanner Muse in the third round, signing undrafted free agent Javin White and adding Bryce Hager on Aug. 7 to join returning players Nicholas Morrow, Justin Phillips and Kyle Wilber. Ukeme Eligwe opted out on Aug. 4.
The 6-foot-3, 250-pound Emanuel has two career interceptions, five passes defensed, four sacks and a fumble recovery for a touchdown at Denver in 2018.
The Raiders also have the maximum 80 players on their current roster so corresponding moves will have to be made to make room for Riddick and Emanuel.