[quote name='ddrpower']How is Dark Crusade purchased? That is, do you receive a CD-key that is considered retail(and therefore available on steam, or at minimum be able to connect with the steam versions?)[/QUOTE]
Dark Crusade CD-keys, regardless of where you get them, can't be activated on Steam. You're thinking of Soulstorm, which is the only DoW game (aside from II, when it comes out) that can have a retail CD-key activated on Steam.
Also, all of the expansions except for Winter Assault are completely standalone, meaning that you don't have to have any other DoW game installed (or even owned, for that matter) to install and play them.