I really wish they would, we would get all the cool missing DLC content, I have both version installed I notice a tad more people on the Steam version lately but it cut down since 2015
Im afraid the DLCs have nothing to do with multiplayer networks.
No, we could not get additional DLCs if crossplay was available.
Under no circumstances it will ever work that you buy a game for one platform and a DLC for another.
These are entirely separated licences. Once the publisher releases DLCs here, they will be available, not before.