Richardson has not played in the NFL's regular season since 2014. He has tried to keep his career alive with short stints in the Canadian Football League and the now-defunct Alliance of American Football.
"[I want to] keep showing myself and my kids that I'll never give up and I'll always follow my dream," Richardson said. "I guess you can say I have a chip on my shoulder."
Richardson recently attended an XFL tryout in St. Louis. He led the AAF with 12 rushing touchdowns before the league shut down in April.
Richardson was the No. 3 overall pick in 2012. He ran for 950 yards and 11 touchdowns in his rookie season with the Cleveland Browns but was traded to the Indianapolis Colts early in his second season.
Richardson ran for 977 yards and six touchdowns on 3.1 yards per carry in 29 games with the Colts.
The XFL is slated to begin play in February.