So, demons then. Pretty much a "change the names and hope no-one notices" approach!
Slaanesh becomes Demoriel, the Twice-Exiled
Tzeentch becomes Coronzom, the Dweller in the Abyss
Khorne becomes Abaddon, the Destroyer
Nurgle becomes Resheph, the White Rider
I envision a host of demons under Satan, all with their own plots and schemes, but these are the ones which easily map to WFRP's Big Four.
Guess I need to write up some factions now!