You can import your .stl file into Meshlab and you can use Meshlab as follows :

1. File => Import Mesh => .stl file.
2. Filters => Remeshing, Simplification and Reconstruction => Subdivision Surfaces: Catmull-Clark - 1 iteration. This will convert all triangles in the mesh to quads.
3. Optionally, Filters => Normals, Curvatures and Orientation => Cut mesh along crease edges
4. Optionally, Filters => Normals, Curvatures and Orientation => Cut mesh along crease edges
5. Optionally, Filters => Normals, Curvatures and Orientation => Re-Compute Vertex Normals
6. File => Export Mesh => .obj file.

Alternatively, you can use Meshmixer to quickly and easily convert your .stl file to .obj and only afterwards import it into Meshlab to apply Catmull-Clark subdivision and the other steps. It will even generate averaged per-vertex normals automatically. When you apply Catmull-Clark in Meshlab you may afterwards need to rebuild the normals anyway.

You can also break a mesh into shading groups in Meshmixer as well, like in Meshlab - by specifying a threshold dihedral angle.