The as-yet-unpriced add-on is called "Trials of St. Lucia," with the second player taking control of St. Lucia herself. The mode's titular trials see players tackling wave after waves of foes, with players able to create, share, and download user-made trials.
Developed by Visceral Games, the PlayStation 3 and Xbox 360 versions of the literary action game hit Europe on February 5 and North America on February 9. A2M's PSP iteration, which likely won't support DLC, will follow a bit later on--around February 23, if the latest retailer listings are to be believed. Thanks to Destructoid for the tip.