If you go to /System -> Library -> PrivateFrameworks -> InstantMessage.framework -> Resources, you will find the .tiff emoticons files for iChat. I duplicated one and changed it a little bit with PhotoShop and then renamed it to veryangry.tiff.
Next I opened SmileyTable.plist within the English.lproj folder in that same location, and I added this to the file:
<dict>Now I have an additional emoticon in iChat!
<key>ASCII</key>
<array>
<string>>:(((</string>
</array>
<key>Filename</key>
<string>veryangry</string>
<key>SmileyName</key>
<string>Very Angry</string>
</dict>
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