I need to know what text to enter into System Prefs / Keyboard / Shortcuts / App Shortcuts, but I can't find the command names in the iTunes menu bar, nor by hovering any of the UI for tooltips.
I don't think you can set this behaviour anywhere in the preferences, but you can make this yourselves using Script Editor (installed on every mac) and FastScripts (get it here).
Building the script:
Open Script editor, copy the following lines to two separate files, and save them both to ~/Library/Scripts/
-- iTunes Skip 5 tell application "iTunes" to set player position to (player position + 5)
-- iTunes Back 5 tell application "iTunes" to set player position to (player position - 5)
Setting the keyboard shortcuts:
Download and open fastscripts. Now you'll find it in the top bar of your screen. Click on it, go to preferences and set your keyboard shortcuts. The arrow keys themselves are not possible to set, because they're already in use for skipping tracks, but a good alternative would be something like: fn + ctrl + rewind/forward
Use ⌘ + Option + (Left / Right) Arrow.