We can host any valid domain extension as long as the domain registrar allows you to change the DNS.
Some registries, Finnish, for example, require DNS zone for a certain domain to be present on nameservers the domain name is being switched to. So it's not possible to add such domain as addon to you cPanel account in a standard way. That's because DNS records are created only when domain is being added as addon, but cPanel does not allow adding domain unless it's pointed to dns1.hostthewebsite.com and dns2.hostthewebsite.com nameservers.
