Is it possible to request an addition or modification of the DNS entries for my domain?

Yes, just submit a support ticket to request the setup of an additional A record for a subdomain referencing an outside ip address, a CNAME to redirect to another domain name or to modify your MX records to point to an outside mail server or other DNS records.

For security reasons, any request to modify a domain record must come from or be verified by the owner of the account.