We cannot import Autocad *.dwg files into OneCNC, so an intermediate conversion (typically to dxf) would be required.
As for solid models, yes, you can tear 'em apart if you want in OneCNC. You need Mill Professional or Mill Expert for this capability. OneCNC is a hybrid modeller, so it can deal with surfaces or solids, but not in a parametric sense. Its not really built for so called organic shapes, but if you find a model is missing a surface, it is typically possible to create what is missing.