What domain extensions can you host?
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 a DNS zone for a certain domain to be present on the nameservers the domain name is being switched to. So it's not possible to add such a domain as an addon to your cPanel account in a standard way. That's because DNS records are created only when a domain is being added as an addon, but cPanel does not allow adding a domain unless it's pointed to dns1.namecheaphosting.com and dns2.namecheaphosting.com nameservers.
Contact us in Live Chat or by submitting a ticket at support.namecheap.com so that we can assist you with adding such a domain to your cPanel.