As far as I know, you can't -- and I've been begging for this since Quark first added the feature. Better, yet, I've been asking for Drop Shadow styles (which could EASILY bee added to Item Styles) to save and quickly re-use settings.
It's ridiculous -- EVERY time I've ever made a drop shadow, I've had to go in and edit settings (e.g., 75% opacity looks nice onscreen, but it's way too dark for print). I use lots of drop shadows (I do lots of display ads for directories) -- thousands of mouse clicks that could be eliminated if Drop Shadows were included in Preferences and Item Styles.
I've never had a response from Quark on this. I love Quark XPress, but sometimes they can be soooo backward in their stubbornness re: leaving in old, counter-productive methods.
Sorry for the rant; this one's a sore spot for me (and don't get me started on how you STILL can't change Runaround on more than one box at a time!!!!)