Yes, you can attach casters to a finished piece of furniture. However, you will need to be careful to avoid damaging the finish on the piece. Make sure to use a drill bit that is slightly smaller than the bolts you will be using, and consider using wood plugs to cover the holes in the plywood after the casters are attached. You may also want to use furniture pads or coasters to protect the finish on the furniture and prevent it from scratching or damaging the floor.