Are these simply just other terrains, or are they imported as a graphical object? I was not sure if importing other high poly terrains would slow performance drastically in a scene.
You can buy a plugin like T4M (terrain for mobile). It lets you draw bushes/trees and textures onto the mesh just like the unity terrain tools, but you can’t edit the mesh once it’s converted (make a backup of your terrain before processing)
It gives you a slider bar to adjust how many vertex points you want to allow for the conversion as well…hence terrain for mobiles.
The OBJ exporter script you can find searching google or the wiki. There’s a $20 colladae exporter in the asset store but that’s for exporting rigs, for static geometry obj is fine. But there’s no “built in” native ability to convert to mesh.