Been there, done that. A facet representation, mesh or .stl, is going to have to be rebuilt. Flat faces are easy enough, but cylindrical, conical or curved surfaces are going to have to be analysed and recreated from scratch, unless your budget stretches to some dedicated mesh-to-surface software. I noticed that in the release notes for 929, Tim says that coplanar triangle facets will be consolidated into faces in Sketchup import. That'd be a useful starting option for rebuilding any mesh or facet format model for editing, even doing it manually is the most logical start to your job, because then you can make more sense of how the curved features relate to them. Cylindrical features are easiest to recreate of curved features, sweeping lines along three point arcs or circles. The hardest thing about rebuilding .stls is that facet representations of features aren't recognised as features, because .stl was never intended to be an editable format, only an easily manufacturable representation of a model. Rebuilding from slices is possible, but often harder work than rebuilding features from vertices.