I can certainly see where such an addition would be handy for some people. It seems maybe all that would be necessary would be an extension centered on the bottom of the spool, something like a 2" ball that would roll along. There is already that knob sticking out for tapping to extend the line, so it might be designed to just snap over that. Many people use a line trimmer for an edger, some prefer a guide while others are content to free hand it.
