CKFinder is a plugin-based application and as such can be relatively easy extended.
Enabling JavaScript Plugins
To add a plugin to CKFinder, upload it to the plugins folder and enable it using the extraPlugins configuration option in config.js:
CKFinder.customConfig = function( config ) { config.extraPlugins = 'dummy'; };
Enabling PHP Plugins
To add a plugin to CKFinder, upload it to the plugins folder and add an include_once/require_once statement in the PHP configuration file (config.php):
include_once "plugins/imageresize/plugin.php";