According to Co-founder and Managing Director of Guerrilla Games Hermen Hulst, Killzone 2 will not be receiving co-op as downloadable content.
"We looked at all the different flavors of co-op early on, but decided our first priority would have to be the single-player experience and the multiplayer," Hulst informed Game Informer. "Rather than just tack on co-op, it would have to be integrated and done to a similar standard as the single and multiplayer."
"Having looked at it again now. I do not think we would want to add it as DLC," he said.
It appears that this is the final word on co-op for Killzone 2. For those looking forward to the new downloadable maps, Sony has confirmed the release date for April 30. The price point for the pack in currently unknown.