Lets say you import a solid square rod that has a special holes towards each of its ends. The part currently works fine for your model. Now you want the rod 2" longer but cannot simply extrude the ends (because of the hole's placement). I'd prefer to be able to extend the part from within. What I tried was spiltting the rod along it's middle plane and extending of the the bodies individually. This affected the body, but not the part. I'm sure I'm doing this incorrectly but I'd really like to know how to extend bodies from the center instead of an outward face.