Addon domains can be converted in the Admin Panel (Domains -> List Domains) Click on the image to covert it to a new account.
As for the main domain, you can always not "Suspend" the main account, but update the config files via SSH to point that account to the suspended page, while leaving the addon-domains to the main account. Most people don't suspend domains, but the hosting of an account. Generally if someone doesn't pay for their domain name, it becomes inactive automatically.