Yes it's possible - you need to edit descr_sm_factions.txt and export_descr_units.txt (there should be some tutorials on the Org somewhere). Just note that if you give a unit the ability to horde (ie give it the can_horde attribute in EDU) then all factions listed in that unit's ownership field must also have the ability to horde. We've also found that there are some problems with playing as a hording faction because there are apparently some files locked away in one of the BI .pak files that are needed for the UI. Not having these files causes a CTD during the loading screen for that faction if it starts as a horde. I'm not sure if it still CTD's if the faction starts off normally, but I'm guessing that it will.

Hope that helps.