Now that I understand your requirements better, here's the quick and easy way to create such a Terrain.
I'm using default iClone props for this example.
First a few props added to a scene:
Next each prop is selected in turn and from the right mouse click menu "Convert to Terrain" is selected for each item:
After all assets have been converted to a Terrain the scene manager now looks like that shown below:
The items are now listed under the Terrain list in the Scene Manager. It is important to make sure that all assets are correctly positioned before converting them to the Terrain, because making texture adjustments is impossible unless the item is re-converted to a prop. Moving, rotating or scaling items converted to a Terrain is also not possible using the Gizmos, this is only possible by changing the values in the Transform panel.
If you have any more questions feel free to ask.
System: Win 10 Pro (1803), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver 417.35), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.