Since you don’t have any information about what the terrain looks like from the outside the boundary in the dwg file, you could use Add Location to import the 3D terrain of the area. You can create the road based on the few points in the dwg file, and then combine them. This way you will have everything in the same place, oriented correctly.
For ease of editing, you could create the terrain using the Fredo TopoShaper extension and Vertex Tools for editing the meshes.