How do I add a custom date comparison operator with RadFilter?

[Todd]

I need to be able to add an "In Time Period" comparison operator option to the dropdown list.  Once the user has selected this option we would display a custom RadFilterDataFieldEditor with a dropdown list of options to choose from ("In Next 7 days", "In Next 30 days", etc).  This would be displayed in place of the built-in calendar control that would be displayed if they chose another comparison operator (such as "Greater than", etc).  How do I add a new comparison operator to the list?  The documentation shows how to remove menu items (Customizing the Menu), but not how to add menu items.  Please advise.  Thanks