Does anyone know if I can play multiplayer with a boxed version of the game (DoW2 Dark Crusade without steam)
DoW2 doesn't have an expansion called Dark Crusade. That's from DoW1. Not sure if the versions are compatible.
Moved to Legacy Discussion.
Adding to what Decepticats said, you might be able to redeem Dark Crusade and play MP with it there. However you might need to submit a support request - https://support.sega.co.uk/hc/en-gb/requests/new.