I've experimented with it a while ago. Trying to add layers of snow on an existing summer texture but then ingame I realized texture was not seamless anymore; the infamous tiling effect.
Making a texture seamless wasn't the problem but then the .tga got off set and looks weird with houses and other objects still on the same place during game play.
The trick with this is first choosing a snow texture that is virtually uniformly white with some random noise or patterns. Then to remove parts of this snow texture by trying to figure out which parts of the original texture would not have snow on it. Typically trees, hedges, paths, roads, the outlines of fields etc would be free of snow, or to a degree less covered. If you do it fairly consistently, with varying degrees of opacity and feathering, then the original objects will still seem to be in the correct place, though - here's a more substantial issue - they will most likely be summer objects, not houses or buildings with snow covered roofs ...
The biggest job is however getting village and urban textures looking believably wintery. Especially with the striking photographic textures used on these maps ... that's a whole different kind of work ...