Robert, thank you for your response.
The alternative 1 seems to be fine, but I think that is too many steps to do. This could be a good way to design a parking lot, but is too complicated to model an existing ground.
I don't understand the alternative 2. I start to work in 3D model with a terrain with only random points imported from txt file. This terrain doesn't have breaklines so the elevations of majority of triangles are wrong. So when I create a linestring by snapping the ends to known random points I still have a problem with the elevations of all other points.
I think that the best way to do what I want is to import random points to dtm, create and modify breaklines with old Inroads tools and then import the dtm to terrain in SS3.
Regards
Wojciech Dudek