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.