Possible solutions:
Terrain chunks/pieces that can be placed wherever (I don’t know if this is defaultly supported) and then build the blueprint around that.
Create a mesh in Max/Maya/Blender and use that for terrain.
Find out how to create a mesh C++/Blueprint and then create the terrain from a heightmap.
I have done a quick test, And have been able to do what I have described.
The only problem is that I need to modify my basic material or something as using the “Visibility” terrain tool does not give visual feedback(Player still falls through terrain).
You should find enough terrain tutorials on YouTube or on the Wiki or Docs.