Well, you can't upload .dds skins because .dds is a texture, where as .jpg is an image. You cannot update textures in any without restarting it. Install new textures on LFS while it's running, and unless it reloads it (Why it would, I have no idea), it wouldn't work until you restarted it.
But it's a good suggestion, although it won't happen unfortunatly.
Maybe the devs can improvise this and create sunroofs for cars, so that everyone could have them. Although nowhere near the top of the list of prioritys, it would be a nice finishing touch.