I think perhaps people are misunderstanding your question.
You need a special piece of software that generates the lightmap as an external image (not part of the mesh/model).
Try SlimShady in the Tools section.
The resulting file is (usually) a bitmap.
For example, if your model is 'house.b3d' then the lightmap should be called 'house_lm.bmp'.
If the '_lm.bmp' letters are added to the basic name of the model then Blitz will know what it is, and automatically load and implement it as it loads your model.
You need a special piece of software that generates the lightmap as an external image (not part of the mesh/model).
Try SlimShady in the Tools section.
The resulting file is (usually) a bitmap.
For example, if your model is 'house.b3d' then the lightmap should be called 'house_lm.bmp'.
If the '_lm.bmp' letters are added to the basic name of the model then Blitz will know what it is, and automatically load and implement it as it loads your model.