You can actually adjust the shape further within the vertex modelling room use the standard vertex tools and the displacement tools. Also you can still use the terrain shaders upon the completed model. Sadly you cannot assign shading domains directly to a terrain in its raw state.. it does need to be converted to a vertex object.

To covert the terrain to a vertex object.. select the terrain then go the top menu in the assemble room and select "Edit" then select "Convert to Other Modeller" at the bottom of the drop-down list, from the new window select "Vertex Modeller" and then click "OK". Now click the Vertex room tool (the spanner at the top of the window" and you will find your terrain ready for editing etc.

Hope this helps..