Rich Cimini of ESPN reported New York Guardians offensive lineman Jarron Jones has also agreed to a deal with Pittsburgh.
Walker, 25, had 19 tackles (five for loss), 4.5 sacks and nine quarterback hits in five games for the New York Guardians.
The Steelers lost nose tackle Javon Hargrave in free agency to the Philadelphia Eagles, making interior depth on the defensive line a priority. They previously added defensive lineman Chris Wormley via a trade with the Baltimore Ravens.
All in all, Pittsburgh has had a relatively quiet offseason, adding players like guard Stefen Wisniewski and tight end Eric Ebron, among others. Solid moves, but not the sort of headline-stealing additions like their divisional rival Cleveland Browns made this offseason, adding offensive tackle Jack Conklin and tight end Austin Hooper.
Pittsburgh will get veteran quarterback Ben Roethlisberger back healthy, however, which should be a major boon for their postseason chances as they chase the Baltimore Ravens.
As for Walker, he had a cup of coffee in the NFL before joining the XFL, spending time on the rosters of the Kansas City Chiefs and Chicago Bears. Rickert told Joe Rutter of TribLive.com that there was "a lot of interest" in the defensive lineman around the league.
"This is what we felt would be like a good fit for him and his ability to rush the passer," Rickert added.