After some playing around, I found a workaround to add your own custom emoticons.
They have to be in PNG, MNG or GIF format. In order to copy someone else's emoticons from Kopete, simply drag the image from the chat window on to the desktop, or some other directory.
Anyhow, find out where Kopete stores emoticons on your system. In the Default directory (for the Default emoticon theme) you'll find a file called emoticons.xml. Simply copy your emoticon images into the Default directory and add the entries into emoticons.xml. An entry looks like this:
<emoticon file="filename.png">
<string>matchstring</string>
</emoticon>
Where matchstring is the trigger text. Make sure that your images are chmoded properly and reload Kopete.. your new emoticons SHOULD be included in the Default emoticon theme.
NOTE: I haven't tried this with anything less than KDE 3.5. If you're not running KDE 3.5, I can't help you (and I suggest you upgrade).