You can get Earthsculptor for free, or buy great IceX Editor. IceX Editor support with A6 levels (WED) and he is really good. With Earthsculptor you can exoprt only heightmap, shadowmap and colormap. This heightmaps you can inport in MED, and saving like HMP terrains file. Now when we have HMP file this is easy. Only what we need is open WED and inport this terrain (HMP file) in your level. For more information about terrains check UserContribution, LevelDesing and Shaders forum.

I hope that helps.
Regreads!