The "center of mass" is set at the origin of the model. Presuming you've got access to the raw file, reposition it's mesh or origin to wherever you want it. Blender, for example, as a "Set Origin to Center of Mass" option, which calculates where the CoM should be. From there, you would edit the mesh independently to "move" the center of mass upwards, downwards, or any other direction you desire. EDIT: Also, I think this belongs somewhere in the Addon Affairs section, as it deals with mods and not the stock game.