oy sorry for letting this go for so long. All the extra content can be picked up via download from Xbox Live. Beyond that, I don't know because I don't own the system.
If you find extra content, or a game that continues SWON, please post. I've looked through Google at length, and can't find a follow-up game, or expansion for the PC game.
It looks like they left it wide open for a follow up when, at the end, Kreiger gets away, and it's just D-Day.