Williams is best known for playing Omar Little in the hit US crime drama, a part he bagged after just one audition, and now he will star as a mystery character named Hawkins.
Other new castings include Andy Garcia as the Mayor and Cecily Strong, Matt Walsh and Pat Kiernan in unspecified roles.
Shooting for Ghostbusters is underway in Boston, with the first on-set pictures emerging last month.
Feig has shared photos of the new uniforms and proton packs on Twitter and will probably keep teasing fans right up until the film's release next July.
Wiig and McCarthy are reported to be playing "unherald authors who write a book positing that ghosts are real" and go by the names Erin Gilbert and Abby Bergman.
McKinnon stars as Bergman's new partner Jillian Holtzmann while Jones is NYC subway worked Patty Tolan who unexpectedly meets the movie's biggest supernatural threat.Reuse content