I don't know of any ways -- and couldn't find any on Google -- to add emoticons to iCal. But if your interest is in using iCal for tracking your moods, I would posit the following:
Create one calendar for each of your moods and assign them appropriate colors. Happy (Yellow), Sad (Purple), Angry (Red), and etc.
Then, on a Happy day, create an all-day-event and assign it to the appropriate calendar. Then, in month view, you'll be able to see all your prior moods at a glance.
I hope this helps.