Go to the following link and download this guy's version of the Infinity Engine Editor called 'Infinity Engine Editor Pro'.
http://www8.50megs.com/gustovm/InfinityEngineGames.html
I've heard it works for BG2 items and spells (I've only tried it for spells so far.. but it worked fine).
You'll have to use Bifftool to extract the item and spell files from the BG2 'data' folder. Biffstripper only works for the first BG. Bifftool comes with a README that tells you how it is done.