Gazebo/MoveIt World Frame

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I am currently using ROS Kinetic on Ubuntu 16.04. I'd like to know if it's possible to fix a robot arm to the world frame in Gazebo without explicitly creating a joint in my robot's URDF to do this. I know it's possible to place a robot arm at the world frame without defining a joint in the URDF, but that makes the robot susceptible to falling during operation. On a related note, assuming this is possible, what is the best way to stop the warnings about 'the 'root link' of the URDF containing inertial values' from displaying in the terminal when MoveIt is launched?