At the moment there is no simple way to do that. To enable fog in all (or selected) maps, you would basically have to create a mod that has a Map Info DED definition for each map where fog should be enabled, setting the "fog" flag in each. The tricky part is that each Map Info definition would have to specify the complete information for each map.
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There is an existing old mod that does something similar: http://files.dengine.net/packs/jdep-20060601.pk3