There are two versions of NSE, classic and 2.x. While classic does have the ability to edit the backsprites, I recommend you use 2.x. You're probably using the "Minisprite" version [Classic]
You can grab NSE 2.X here or in the toolbox.
Load your ROM in NSE 2.X, then click File>Navigate.
To the right, in the Navigate window, is a grey folder called POKEMON Open it, then open to BPRE/BackSprites.
There are folders containing all of the backsprites in firered.
NSE 2.X lets you edit all of the back-sprites and their palettes directly within the interface, no need to import or export anything!