According to TV Guide, the pair have both won recurring roles in the drama.
Willig is expected to play Special Agent Simon Cade, who is said to be friendly and well-educated with a photographic memory.
Willig, who used to play in the NFL, has previously guest starred in shows including Dexter, My Name Is Earl and Chuck.
Meanwhile, Ballard will star in NCIS as Special Agent Gayne Levin, who is pleased to be back in Washington after being sent on various assignments.
Ballard played David in Numb3rs and has also worked on shows including Sabrina The Teenage Witch and Dark Angel.
Simon and Gayne are expected to join the show to help the team track down a serial killer, although reports have suggested that they may not get on with the established characters.
NCIS airs on Tuesdays at 8/7c on CBS.
> CBS renews 'NCIS' for ninth season