Can I set up another ALIAS record pointed to my existing host mapped Help Center URL?
You can map only one CNAME entry against your Zendesk Help Center URL address. You can't set up an ALIAS record - for example support.yourdomain.net to point to your existing host mapped subdomain - for example support.yourdomain.com . This is due to security reasons and to prevent any DNS spoofing.
The ALIAS record maps a name to another name, but in turns it can coexist with other records on that domain name.
For more information, see Changing the address of your Help Center subdomain (host mapping).