MoveIt! execution of joint group trajectories with "all joints controller"

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I have a robot with a couple of joint groups configured. The controller, however, serves a JointTrajectoryAction for the whole robot (there are no individual controllers for each joint group). My question now is: Is there a simple way for converting a trajectory that has been planned for a joint group to a trajectory for the whole robot, containing all joints? The resulting trajectory should just keep all joints that are not part of the planning group fixed. I know I can easily program that myself but it seems to me that this is a useful feature and might be implemented somewhere.