If WED doesn't have a button for it you can parse the .wmb format which has a 'lightmaps' member and contains the pixel data for the lightmaps generated by the map compiler.
You could simply write out these pixels to an image file, and you have your lightmap. Each lightmap is 3MB (3 * 1024 * 1024).
It's possible, but you need some programming knowledge. You must write the light map exporter, either in lite-C or in another language. The WMB format is described in the manual.
