You dont have that spell available in game?
I dont know how to edit save game file to add some specific item/scroll etc in game already started.
But you can do next :
shift key +
tilda (~) , little chat window will open, there type `magicbook` and press
enter. This will give you all spells in game in your spellbook, and 7 scrolls of every spell. So then you can destroy/sell scrolls (gold was never big problem in this game). As for spells you get and didnt want them ... well, dont use them. I have no other advice for this particular problem
