Message icons can be added just by uploading them into the icon directory and keeping the naming scheme (ie. icon##.gif).
Graemlins are a little harder to add, and you'll probably want to get the hack that adds a graemlins control panel. Look for it in one of the v6 Mods forums, either Beta or Finished (I'm guessing Beta, but I can't remember exactly right now.)
---Skorpion