Position Sun Anywhere?

Within the current implementation, no, it is not possible. SketchUp wasn’t designed to be a renderer, so it doesn’t include much flexibility about placing lights to illuminate the geometry. The only choices are a light at the camera and the sun, and the sun position is calculated based on real-world location and time.

This might make a reasonable feature request (though who knows if it would get any priority), but for now your only option is to use a real renderer that includes lights.