It scares me when i can't do it directly, too, since one might assume this is an important feature for architects.
Here are the steps:
Make a 3D TOP VIEW using the internal engine in Analytic hidden line mode calling for Vector Sun Shadows - no contours.
MARQUEE [THE FLAT 2D MARQUEE] the image and COPY. You'll get the attached dialog box. DO your copy accordingly, selecting only the shadow edges.
PASTE this into your plan view. It will be accurately positioned and scaled. Be sure to have nothing else on the Archicad layer in order to simplify selecting the shadow edges.
If you want the shadows to be fills, COPY from a SHADED Analytical internal engine view. Alter the copy dialog to COPY shadow POLYGONS.
When you PASTE the shadow polygons into the PLAN view, they will be all different colors corresponding to what they were in the 3D view. Since they are pasted into the Archicad layer, you can select them all at once and change them to the appropriate shadow color.
This is what i remember - but you may need to refine the process somewhat.
Dwight Atkinson