If you are using OpenRoad Connect Edition, you can select what you need to obtain when processing corridors creating new feature definitions, and selecting different display settings. You can obtain Top Mesh, Bottom Mesh, Components, Linear Features, include or not Null Point Lines, Major Contours, Minor Contours, or any combination of them. If you create a Feature definition for a corridor named "Top Mesh" and set only Top Mesh in display settings, you will obtain the Top Mesh, after that you can create a terrain model from elements selecting the top Mesh as break lines, this step can be done creating a new file with a 3d seed, attaching the 3d model containing the top mesh; once you have your terrain model, you can create another file with a 3d model, then you attach the Existing groud model and the top mesh model and create your complex terrain.