The SDK also comes bundled with a compatible version of blender. That is the 3d modeller of choice, though it is possible to use others, even Google SketchUp. Support/documentation may be patchy in that instance.
Depending on the size of your model, for example if it’s a large terrain, you will want to split it into chunks, a lot like Minecraft does.
You cannot modify j3o files, they are jmonkey data files that cannot be reversed back. You import your models and they become j3o. You need the original model.
I have checked this tutorial, I also checked the terrain editor too, and I saw that I can create a grass plain, I can add second texture, but my problem is that I cannot add road / asphalt texture with sharp edges, but only smooth contours.
I have recently create some mesh with blender a.n.t landscape than I create heightmap from it if thats helps… Of course you can always create some mesh with blender and convert to j3o and add MeshCollisionShape in PhysicsSpace for example…
Most games dont rely on their terrain system for the actual roads, and instead generate a road mesh that closely follows the terrain.
There isnt a built in system for this in jme, but it is easy to generate a mesh and extrude it along a curve. Youll need to write the mesh generation, curve and your own editor though.
Optionally you could make the road in blender and import it, it would be less intuitive but will get you going sooner