You can change the default predefined styles in HTML editors in SharePoint.
Standard MOSS HTML Editor.
You can specify a unique CSS class name prefix for each publishing HTML field control so that you can present different custom styles for different sections of the page. Look here for full details how to: http://msdn2.microsoft.com/en-us/library/ms551040.aspx
Telerik HTML Editor
You can add the custom heading styles to the CssEditor.css file. This file should be stored in the \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\
4.1.0.0__1f131a624888eeed\RadControls\Editor\ folder. If the file is not present there, simply create a new one.