For static geometry any of the popular packages are okay as long as they export to dx8.1.
There have been tutorials, just do a search.
Importing your own maps isn't really the done thing in FPSC, as it's built around the editor which is one it's main selling features.
You can import your own geometry, but this is more for entities and enemies. That said, bringing in your own maps is possible, maybe, it's just not a straightforward process yet.
I don't think anybody around here has figured out how to achieve this yet at any rate.
You can use the segment editor to build your own segments. It all depends on what you want.
You really need to download the demo and have a play, I think.
instinct is more valuable than intelligence.....