It looks like Landscape actors created with World Partitioning enabled cannot be just copied over to a non-partitioned level. I also tried exporting and importing the landscape with its own tool but still nothing… you just get an empty landscape object.
My gut feeling is that all the data in the world partitioned landscape is stored in children actors or different components, thus when you bring your data in a non-partitioned world, there will not be a data structure available to store that data.