While I don’t know of any way to import a 3D object as a terrain, since terrains use 2D images, you can “import” an image to use as a starting point for your terrain and then use the terrain editor. In your favorite image editor, load your image, resize it to match the dimensions of your terrain canvas and make sure it’s a .png or other valid format, and copy it to the Windows clipboard. In Bryce, create a random new terrain, then in the terrain editor, use <ctrl>v to paste a .png image over the top of it.