You can use any external terrain generation software capable of exporting to RAW or Leveller file formats. You can also use the Heightmap Extension Interface source code we provide in your downloads page to build and compile your own custom terrain import pipeline.
Here, however, is my broken record advise. Please don't dive into external massive procedural world generation on day 1 in the Cloud. Get to know the tools. Build some game systems. Build a couple of test zones. Setup seamless 2.0 areas for a few weeks. Learn the world you are going to build, and ensure that the path you are on to get there is solid, then dive into external and procedural terrain generation. I know it seems fun to have thousands of square kilometers of game space instantly made for you, but it's just not that simple, so explore and learn and practice first: You'll thank me for it later.
