i think there is multiply, darken, and a whole bunch of others, i was told that 5 (2xmodulate) was the best for lightmaps...
Well that really only applies if the program which creates the lightmap is using the same blend mode. Since Slim Shady is based on YAL, which I believe only use multiply blend mode lightmaps, you should really stick to multiply.
But frankly I don't see why you're trying to apply a lightmap to a pre-lightmapping model. As Joe siad, the point of using a lightmapper is to export your scene with the lightmap together as a b3d.