Cox was drafted by the Cardinals in the first round of the 2010 Draft. He stayed in the organization until the middle of the 2012 season, when he was traded to the Marlins for right hander Edward Mujica.
Cox, who played college ball at the University of Arkansas, was scouted by Nationals scouting director Kris Kline.
"The left-handed bat appealed to us," Scialabba said. "To be able to play third base and add him to the inventory of left-handed bats in the organization and provide some depth -- that is something that we were targeting going into this phase.