That depends on the program you are exporting from, but for Terrain generators like World Machine, you would actually export your terrain as a tiled heightmap. You can then import these tiles and use World Composition to arrange each tile to match up with one another. If you were trying to work with a smaller landscape, then you can simply import a heightmap and sculpt on top of that mesh itself. Once it has been imported and essentially stamped as a Landscape actor, it can be sculpted and painted on just like a regular landscape actor.
Cheers,