Titled 'The Doctor Who Cloned Me', the new single-player mission features additional weapons, enemies and bosses as Duke hunts Dr Proton "deep in the heart of Area 51".
The DLC, which is available from December 13, also contains extra Achievements and Trophies, as well as four new multiplayer maps which can be played across all of the game's multiplayer modes.
The maps, titled Sky High, Command EDF, Drop Zone and Biohazard, enable players to battle it out in locations such as a Las Vegas office block and the rooftop of Duke's Lady Killer casino.
The DLC will be available from next Tuesday for $9.99 (UK pricing to be confirmed) on the PlayStation 3 and PC, and 800 Microsoft Points (£6.80 / $10) on the Xbox 360.
Released in June after 13 years in development, Duke Nukem Forever topped the UK all-formats chart in its debut week of release.
Watch the Duke Nukem Forever Shrinkage Trailer below: