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I am looking for the answer that is possible to import an object then make it terrain and paintable to create mountains etc. on its surface?
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 v to paste a .png image over the top of it.
As Sean said, use a picture, 16-bit grey scale TIFF, like DEMs come. You can then edit it in the TE. If you have an object, you can render it from top as Distant Mask and export the render as picture. There is no 16-bit grey scale option, unfortunately. (1) you export as 24 bit TIFF or BMP. When importing into the TE, there will be steps because of the low resolution. This can be smoothed in the TE to some extent. (2) you can alo export the rendered Distance Mask as 96-bit HDRI. You will need a graphics program to open it and convert it to a 16-bit grey scale TIFF.