sunlight is actually kinda awkward really. at least shadowmapped ones. logically the light needs to be an infiniteish distance away, with infinite radius. of course, computers don't do infinite, which results in some really nasty precision issues if you try doing sunlight with some generic rtlight.
the solution, then, is to constrain the near+far planes of your light to either contain the frustum (or part of it if you're using some weird multi-shadowmap approach), or it need to contain the entire map (nearest point to furthest). but in doing so, you ensure that only one shadow axis needs rendering instead of 6.
neither fte nor dp actually support such special-case lights.
a dynamic light can be added using ef_fulldynamic/pflags stuff (api derived from tenebrae). this will allow you to move an rtlight dynamically. you can also change the intensity with it too. expect problems with lights needing to be nearer than the sky.
if you want it to work in any engine, you can just use lightstyle creatively to change lighting over time. By using separate morning/evening lights that are changing indepndantly, it should be vaugely possible to at least partly hide the fact that its not actually moving. You'll also need a light util that can cope with multiple suns with independant lightstyles.