You can export terrain geometry using a Micro Exporter. Texture data can not be exported at this time. If you make changes to the model in 3DS Max/Maya and reimport into TG2, it will behave differently from a native displaced terrain, so that's not really an ideal approach to terrain editing. For explicit terrain modeling in TG2, you can try a Painted Shader along with a Displacement Shader, or consider using a dedicated heightfield modeler like Leveller.
- Oshyan