Lightmapping can certainly be done in an external program. For this to work you'll usually need to have two uv-sets for each model that gets lightmapped -- this is because they will usually have tiled/repeating textures, but you don't want the lightmap to be tiled.

I don't know if a default shader comes with this, but I'm pretty sure a shader will be necessary to make use of both the lightmap and the other textures.

As far as culling goes, I think it's always a good idea to cull some things manually anyway, and BSP culling generally won't do much for an outdoor level.

Jibb