Jesus, I found the solution 5 minutes after writing this, lol! The controls in BTB are complex (click & drag in surface window).
Anyway, I made whole track layout and added terrain. And after couple of tutorials from BTB site, I saw one very good solution how to make grass, hills, ditches etc. Instead to add terrain by click and drag or add it to whole track at once (via option in BTB), I can add many cross sections on the track and add grass material. It's a way better and simplier solution than editing the terrain polygon vertices, it's a headache sometimes.
What solution do you use? Some tips?