Difficult. Proxy Pass is a third party app and the developer has not maintained it for years. You can set the immutable bit on the file - "chattr +i /etc/httpd/conf.d/x_proxypass.conf" so the file will never change, but you need to remember to unset it when you need to change it. Otherwise have a look at hacking /usr/clearos/apps/proxypass/libraries/ProxyPassProcessor.php to get what you want.