The most convenient way to convert 3D meshes from OBJ fromat to SLDPRT format is to use SolidWorks and simply import the model for further modeling, which will essentially allow direct obj to sldprt conversion. A viable alternative would be to use some other 3D modeling programs or dedicated 3D converters like CrossManager.
Open OBJ file Open SLDPRT fileA set of 3D modeling tools
3D CAD/CAM tool for product development
A CAD converter with huge format support