Call a script after user add

Yes. See interface/lib/plugins/

I use a similar plugin in the billing module, here a first few lines to get an idea how to bind your function to the events:

Code:

class billing_plugin {

    var $plugin_name        = 'billing_plugin';
    var $class_name         = 'billing_plugin';

    /*
            This function is called when the plugin is loaded
    */
    function onLoad() {
        global $app;

        //* Events
        $app->plugin->registerEvent('client:client:on_after_update', 'billing_plugin', 'client_update');
        $app->plugin->registerEvent('client:client:on_after_insert', 'billing_plugin', 'client_insert');
        $app->plugin->registerEvent('client:client:on_after_delete', 'billing_plugin', 'client_delete');
        $app->plugin->registerEvent('client:client:on_check_delete', 'billing_plugin', 'client_before_delete');

        $app->plugin->registerEvent('client:reseller:on_after_update', 'billing_plugin', 'client_update');
        $app->plugin->registerEvent('client:reseller:on_after_insert', 'billing_plugin', 'client_insert');
        $app->plugin->registerEvent('client:reseller:on_after_delete', 'billing_plugin', 'client_delete');
        $app->plugin->registerEvent('client:reseller:on_before_delete', 'billing_plugin', 'client_before_delete');

And an important note! The interface plugins are read into the session at login, so when you create a new plugin, then logout and login again to activate it You don't have to repeat that when you change it (unless you want to register it for additional events).