GOW3 is done, they haven't release any DLC since a year for it, and even tough there is still a decent pool or players, no new content will ever be added. Also i don't think Karn could be added to Overrun, as its not only just a skin in that mode, its a whole character with aptitude.
And yeah why C.Carmine is not in the game(he is, just not unlocked) is beyond me... This sound DLC for sure.
Gears of War Legend