Nothing fancy, but nice. Control-click on the Skype icon and choose Show Package Contents from the pop-up menu. Navigate into Contents -> Resources -> Emoticons. Control-click on Animated Emoticons.SkypeEmoticonset and choose Show Package Contents again. In the new window that opens, down at the bottom you'll find Emoticons.plist. Open it with a text editor (back it up first), and make sure Skype isn't running.

Now scroll down to the end of the document and find the first emoticon with the line <key>Hidden</key> (starting after the emoticon "star"). Simply delete the lines...

<key>Hidden</key>
<true/>

...under every hidden emoticon, save the file, and you're done. The hidden emoticons will now show as a regular option in Skype. The Emoticons may vary depending on your version of the program.