If you registered a domain with a non-ASCII character set, cPanel will give you an error if you try to add it to your account as an add-on or parked domain.

But that's okay. There's a workaround: PUNYCODE! Go to NameISP's Punycode converter, type in your domain name and click Convert. Then you can copy that into cPanel's add-on or parked domains. Voila!