Hey Alex
I never heard back from you, but I learned a little bit more since then.
Open 2 nifskopes, one with a bounding box and one without. On the render drop down, up top, you can check "show hidden" to see the bounding and selection boxes.
On the nif with the bounding box, find and select the box's top branch (should say "ninode bounding box"). Its attached to the armature master. Right click and copy branch.
On the other nif, select the armature master (IE bip 01, or whatever its called) and paste branch. The box should appear attached to the armature.
You'll need to find a good fit between the two. I know how to resize a box in Blender, but i've forgotten how to do it nifskope.
RD