I doubt you can safely and reliably add new stuff to the game until the CK is out, but in the meantime you could modify existing meshes to achieve the same effect. Thanks to the layered armor system, anything you add to those pieces will be on top of any underlying armor, making them well suited for adding backpacks. I was thinking of something along the lines of Leather backpack or Jokerine's lovely Sackpack. http://i.imgur.com/TSEEm3R.jpg In New Vegas backpacks couldn't have straps or they would clip horribly with most armors, but if you attach a backpack to the leather chest piece in Fallout 4 it will fit perfectly!
