Hi Simon,
There's no easy way to do this in your version. Some people have hacked the PHP scripts, but this is not really supported.
With the latest release of WiretapCentral (2010) just announced at NAB, you can edit an XML file to add/modify/remove presets to your heart's content. We have also provided additional presets, including an improved H264 export.
There are a ton of other features and enhancements, so be sure to check it out.
Cheers,
Dan