Quake3World

I modelled a few small and plain rooms in blender and exported them as an .ase file. After setting "spawnflags" to "6" everything was fine. I can walk trough it and it has lightmaps. It still needed a big brush box around for vis.

Is it possible to make this imported geometry structural or does the compiler only understand this confusing CSG lego?

No, models are not "structural". You will still need to pop in a hull and other vis-blocking structures.

You may also still need to do some manual clipping of any complicated or non-axial stuff (just to be sure) and definitely do some bot clipping.