The only fog supported by DarkPlaces engine is global fog. It means that it does not support anything related to brush/shader fog.
Global fog is set in Worldspawn entity or in mapname.mapinfo file:
fog <density> <red> <green> <blue> [<alpha> <mindist> <maxdist> <top> <fade-height>]
density - how much fog to add per distance, this is virtual value. 1 means very strong fog.
red - red component of fog color
green - green component of fog color
blue - blue component of fog color
alpha - fog effect opacity (default is 1)
mindist - distance to start applying fog at
maxdist - maximal distance apply fog at
top - origin of height plane at worldspace (game units)
height - fog height fade in game units