Hey there, I'm also a nifskope modder. Yeah that's true about the permission thing. A lot of the things well all of them really, that I have made with through nifskope I was unable to upload because I could never get permission, I did ask but no one ever got back to me on it so I just never uploaded. As for making great things its pretty simple, if your just doing a mashup with that duster and making different variations, then all you have to do is this really.
Open up the nif that has the part you want, right click on the object you want, and it should highlight it to show you what your targeting, and go down to "block" then hit "copy branch" then open the nif you want to add that part to (make sure to keep that other nif open though), right click "scene root" and then go to "block" and hit "paste branch." The part will then be added to this nif. It also helps to very closely examine the part your adding to the duster to make sure that it will actually fit onto it without clipping. I'll also add that even if something doesn't look like there'll be clipping in nifskope, the game might show differently especially when the armor is animating like running or jumping.
Doing blender also helps, like Limitless mentioned, because through it you can easily take something off a mesh or separate a part from the base mesh so that you can remove it in nifskope. Because some nifty looking armors out there with all their pieces are actually one solid object, therefore making it so you can't take that piece you want without also taking the whole thing. Anyway hope you find some of this helpful and good luck to you.