Metars sky conditions use a ceilometer or similar equipment to determine cloud heights. You could use the solar to determine cloud cover % and let WD figure cloud height (already does this btw). But visibility…hmmm, thinking to myself…I do have a piece of equipment (see this thread) http://discourse.weather-watch.com/t/10406 that will measure the amount of dust/smoke/whatever in the air. You could do manual estimations (or compare with local metar conditions) to create a table that correlates the amount of crud in the air with expected visibility and then manually enter that into WD daily.
So it could, in theory, be rigged to provide most of the data…
Also, I don’t know why…for some reason my local metar is in the synop directory. When I get home I will look into how I managed to get it there instead of the metar directory…so I assume you can tell WD to store it someplace else…