If the terrain tool doesn't work, MeshLab (very good open-source mesh manipulation app) might. As for turning recalcitrant surfaces and meshes into solids, there are a few workaround methods. A mesh or surface can be exploded down to polylines, the polylines given thickness and then property-converted to solids en masse, and then, while they're still all selected, multi-added together. Only thing to be on the watch for is that surfaces tend to have some facets with inverted normals, and when given thickness, they thicken in the opposite direction to the others, the remedy is to apply a negative thickness to those polylines. A mesh converted to zero thickness solid can also be added as a face of an
open a box that's composed of surface faces, rather than being a volumetric solid (so long as the mesh/surface overlaps the sides of the open box to enclose a volume), then use facet editor to pick the appropriate faces to format/modify a solid|from faces.