That is easy, I've done that in my mod.
All you need is a root template with a unique icon. For each different backpack, you need a different root template, because icons are defined with the root template. If you only want to name them differently, you need to create multiple 'links' between a root template and a 'stat' and give those stats different names.
It is probably more difficult to create good looking icons, at least that was the case for me ;-)
It's another thing to change the appearance of backpacks in the world, how they look when outside the inventory, that's far beyond my abilities and I used existing graphics of other things for that.
You can download Abraxas' 'Better Logistics' or my 'XC_Bags', extract them and have a look at how we did things.
To extract both, use Norbyte's extractor tool. (Both are in zip files if you download them from Nexus.)
