Perhaps you don't want the trouble of sculpting the terrain using the tools offered in Lumion. Truth be told, in some situations, it is easier and more productive to model the terrain outside Lumion and import that terrain along with the building. Lumion has fantastic material that blends the terrain we imported with the landscape.
Before you begin, it is essential that you model a terrain in your favorite 3D modeling package and import it in Lumion.
Select the Edit Materials button and click on the 3D model you imported.
After selecting the 3D model, you need to pick the Landscape material found in the Custom section, as you can see in the following screenshot:
Apply this material, and the terrain you just modeled will look like part of the Lumion terrain.
You can select the Paint submenu and paint on top of this material by adding a different material...