Add's cPanel integration to WPMUDEV's Domain Mapping
This will hook into the cPanel API and automatically park a domain that is mapped through Wordpress. It'll also unpark that domain if it's deleted.
Upload to wp-content/plugins/cpanel-domain-mapping/ then go to your network plugins panel and activate it!
You can then go to Settings > Domain Mapping cPanel and enter your cPanel settings so that the API can connect.
After that, you're good to go!