Mousetrap: Do you mean the sort of "frequently used" smileys? (That you get by clicking the smiley beside the word "Message")

If so, I was able to add one. I clicked "Change Smileys"
It takes you "Javascript" tab of your settings.
Scroll down and click "Add Smileys"
Then you see some lines of all the smileys that you don't yet have in your "frequently used" ones.
Click on smileys you want to add
Click Save and Finish.

If that's not what you meant, then I'm sorry.