modX::addExtensionPackage(

gistfile1.php

<?php
    public function addExtensionPackage($name,$path,array $options = array()) {
        // What is it for?
        // $extPackages
        // It`s not using
        $extPackages = $this->getOption('extension_packages');
        $extPackages = !empty($extPackages) ? $extPackages : array();
        $extPackages = is_array($extPackages) ? $extPackages : $this->fromJSON($extPackages);
        $extPackages[$name] = $options;
        $extPackages['path'] = $path;

        /** @var modSystemSetting $setting */
        $setting = $this->getObject('modSystemSetting',array(
            'key' => 'extension_packages',
        ));
        if (empty($setting)) {
            $setting = $this->newObject('modSystemSetting');
            $setting->set('key','extension_packages');
            $setting->set('namespace','core');
            $setting->set('xtype','textfield');
            $setting->set('area','system');
        }
        $value = $setting->get('value');
        $value = is_array($value) ? $value : $this->fromJSON($value);
        if (empty($value)) {
            $value = array();
            $value[$name] = $options;
            $value[$name]['path'] = $path;
            $value = '['.$this->toJSON($value).']';
        } else {
            $found = false;
            foreach ($value as $k => $v) {
                foreach ($v as $kk => $vv) {
                    if ($kk == $name) {
                        $found = true;
                    }
                }
            }
            if (!$found) {
                // And this? $extPack
                // It`s not defined
                $extPack[$name] = $options;
                $extPack[$name]['path'] = $path;
                $value[] = $extPack;
            }
            $value = $this->toJSON($value);
        }
        $value = str_replace('\\','',$value);
        $setting->set('value',$value);
        return $setting->save();
    }