You can download the file from: HERE
Start by creating the terrein using Sandbox Tool > From Scratch, with a big value of Grid spacing
Then, in the areas where it overlaps with your land, add details with help of Sandbox Tool > Add Detail
Continue with the use of Sandbox Tool > Smoove to give the desired shape to the terrain.
Be careful that the new terrain intersects the one you had.
Finally, use Paint Bucket Tool to retrieve texture from the satellite terrain and apply it to the new terrain.
