I'm using 3ds max as I can do wgt because I'm exporting texture and .sco
You need to do the following to get your backpack working. If it is not attached to a bone there will be no wgt. Every engine does rigging slightly differently so i can see the confusion xD.
1. Select backpack and position it on the character, go to Hierarchy, then select "Affect Pivot Only" and set the values to zero .
2. Go to Modifier List in Modify, and select "Skin". You will see this, click "Add" and add the bones that the backpack will affect. For example i will select Spine 3 and Spine 2.
3. Now you can see the backpack is attached to the biped.
4. Now you need textures for the backpack, so go to Rendering (Top menu) -> Material Editor -> Compact Material Editor. Click an empty texture map and select the box right to Diffuse, find the texture, and drag it onto the backpack it is now textured. Make sure to name the texture here so the material file isn't called **_-_Default.
5. You can view the texture by going to Rendering (Top menu) -> Render
6. Export the bag with TSG utility manager and make a normal and specular map for the model, and apply it in the .mat file. You can also edit it in game with the Material Editor and change the Metalness, Spec, Spec Power, as well as the intensities, etc. Good luck with this, this mini tutorial also applies to body armor, helmets, gloves, etc. DONT forget to go to Physx Editor in studio and create physics for this!
If anybody wants a tutorial on how to add new body part ascessories (Scarfs, Gloves, Leg Armor, i will make a tutorial later