The icons are packed into MT_COMMANDBAR.TGA and MT_COMMANDBAR.MTD.
Use my MTD Editor
tool to manipulate these two files.
Just create the icon(s) you want in a seperate TGA file, with alpha mask and everything. Then use my MTD Editor to insert them into the MT_COMMANDBAR file pair and you're done.
Oh, and make sure to put the altered MT_COMMANDBAR files in the proper directory. i.e., GameData\Data\Art\Textures, or somewhere in the Mod Path.
PS: You may see tutorials floating around on the web that talk about using MTDEdit and remembering coordinates and hex editing and stuff. Ignore them.
MTDEdit is another editor that's not as sophisticated (as blunt as it may sound) as my editor.