Can I save a MultibodyPlant into URDF (or other supported xml type file)?

For example I can build a model using functions like AddRigidbody and AddJoint. After the model is built, can I save the model into a URDF file or other xml type file? Drake can load model from URDF or SDF using parser but I haven't find function for saving the MultibodyPlant into URDF or SDF. Thank you for your answer!