Lightmaps are greyscale textures layered on with multiply blending. Lightmaps can be generated using code from the Code Archives like YAL or you can simply use a tool with lightmapping functionality like Maplet or Lightbulb.
Did you mean additive blending? You are probably right as I don't know much about lightmaps and such, and I've still no idea what muliply blending is used for. :O