Can I use STL files to create solid geometry?

Yes, there is a check box in the Options menu under the File options section for STL files: Import as body. Check this option on to import STL files as solids. This imports a solid with the same faces as the STL mesh and you can modify the geometry the same way you would edit solids created in SpaceClaim. Check ON the Merge Faces option to merge the mesh faces during import