It appears that there are several options how to actually achieve cob to stl conversion. Both are 3D/CAD file types so you should have no trouble with converting Caligari 3D object files (.cob) as Stereolithography files (.stl). Tools like Accutrans 3D or CopperCube can be used for great effect in order to convert cob to stl.
Open COB file Open STL fileA CAD 3D geometry translation software
A free online 3D model conversion service
A 3D CAD translation software