Rather than dumping a whole scoop in at once, gradually introduce the additive feed by feed, so start with a sprinkle and gradually build up the amount you add until you're feeding the recomended amount.
Will she eat molasses? If she does, mis the additive in with some bran, put that into some hard feed and then put molasses into it too. Usually that'll get them eating it.
Haha don't worry, I totally understand about fussy eaters. My old horse would only eat lucerne and one type of hard feed. WOuldn't touch apples or carrots. Although she did have an odd liking of sour cream and chives flavoured chips :S