The channel menu is implemented using DHTML layers. The Javascript API for creating and manipulating menus is implemented in popup.js which can be found at PortalServer-DataDir/portal/portal-ID/web-src/enterprise/js directory. This is the deployed copy of the Javascript file. Any changes to this file should be made here. Then redeploy Portal Server using PortalServer-base/bin/psadmin redeploy (Solaris).
The menu Javascript API provides methods to create new menus, add menu items, and specify actions for each menu item.