Can I use custom meshes as terrains? turning them into terrain objects?

I’ve been trying to find a way to cut apart then height map without using the cutout tool in the unity store. I’ve also tried using my own meshes and then using that mesh at a terrain object. is there a way to use unity trees on a mesh?

Use Custom DEM data to create real world analog.

The issues I’m having is that all of the Lakes and rivers are black on the DEM data. and the terrain objects wont allow you to go below 0. in the Terrain editor. any Ideas?

If you are russian speaker then video for you: Unity Engine Roads and walls with 3ds max and Unity - YouTube
Else use this scripts: http://wiki.unity3d.com/index.php/TerrainObjExporter
http://wiki.unity3d.com/index.php/Object2Terrain