Well I've just added the Slavs to my game. First I gave them a settlement, then I took away the settlement and they start as a horde. So any faction that can form a horde, can start a campaign as a horde. And I've been able to add the horde ability to several factions so somewhere along the way, you've possibly done something wrong if it isn't working.

If you've copied the right bits from the descr_sm_factions file, added the horde unit (and given him the can_horde attribute) and something still isn't working, then off the top of my head I can't say why.