Here's a trick borrowed from MMORPG models on how to easily attach objects to a MMD model (usually the hands). It makes holding x.file models of guns, bags, microphones, and other objects that can be held in the hands a breeze. You will no longer need dummy bones to do this and the attached/held object moves with the hands.
You can extend the procedure by adding additional attachment points for carrying (for example) a backpack or sword across the back.
Hope this helps. If you have questions please leave a note in the comments below.
Miku and leeks are the default models located in their respective directories of your MMD folder.