Ok its pretty simple,
Open the character file, there is a block list column, with all the NiNodes, open one, for example select with the mouse Bip01 LHand, with that selected go in file on the high left corner of the screen, go on import, import.Obj , select Ok to the question, select an object that you have exported , click open, there you probably see the object now, click on the object imported, now you see that is selected even in the menu, rename it weapon (double click on the name in the menu selection to rename), if you click on the object on the screen with the right mouse button it will open a menu, choose transform, then edit there you could scale and move the object, obviously save the character.nif when done (make a safe copy of your old character before change it. ) Probably the worst tutorial ever but im not to much a tutorial guy.