Load the clothing item and in the setup room click on spawn props, then leave the setup room and delete the figure but NOT the props. Load the human figure and in the joint editor click on zero pose. Then what you do is select each body part and click on replace body part with prop for each part of the clothing. This will only work for entirely covered body parts, it wont work with a skirt because the thigh parts will be replaced by the clothing item and it will look wrong. Remember to not move any of the clothing parts before you replace the figure parts or they will end up in the wrong place.