@Ventilator: A lightmap does not need its own "seams". Any place in the UV mesh corresponds to a XYZ value to calculate the lighting for, so as long as UV mesh parts don't intersect, the content of the lightmap is completely defined by the XYZ values of its UV coordinates.

Most lightmappers first unwrap the UV mesh, then apply a linear transformation (u' = a*u + b) to the UV coordinates to map them into the lightmap space, and last add an uv offset to ambiguous mesh parts (same UV but different XYZ). I don't know of a lightmapper that would create a completely independent UV set, but this is theoretically possible.

For importing an already-existing lightmap that does not contain duplicated vertices, you just need to go through all uv values that are not already set, and then set them to the uv values of the next vertex in the mesh that has the same xyz coordinates.

Hope this makes sense.

@William: This sort of discussion leads to nothing. If you really want to write a lightmapper or a lightmap importer, I think you need first some understanding of meshes, UV coordinates, DirectX, vertex formats etc. Otherwise I can see that you feel "left out in the blue", but can't help it.