gazebo robot model modifying

Dear Alruists, I have to modify some functionality of a relatively complex robot (legged & wheeled), which I did not work before. I can spawn the model(urdf, idont have xacro file) and change joint angles in rviz roslaunch urdf_sim_tutorial gazebo.launch model:='$(find urdf_tutorial)/urdf/tt3.urdf' - . I want to add force torque sensor to calculate efforts on wheel (added below portion but it shows > [FATAL] [1515084753.020957716, 0.114000000]: gazebo_ros_ft_sensor plugin error: jointName: force_torque_sensor_rear_right does not exist )true100.0ft_sensor_topicforce_torque_sensor_rear_right - Move the robot on that flat surface (so added transmissions, but is differential drive applicable in wheel which are in different legs? ) - Control pose from topics can you suggest any easy solution for this? Seeing some tutorials it seems modifying xacro, it would have been easier, but I dont have this, so have to add all manually.