The DNS Access Layer is not meant to be used to force the ProxySG appliance to resolve specific host names to specific IP addresses while processing forward or reverse proxy web traffic. It is meant to be used when using the ProxySG IP as a DNS server; at that time, policies on the DNS Access Layer will start to apply.
The ProxySG doesn't offer the option to add or edit anything like a hosts file in a Windows environment. The ProxySG appliance must query the listed DNS servers for every request, unless it is already being cached on the DNS cache.
To create customized specific DNS mapping, add a local DNS server IP to the DNS server list configuration, then configure this DNS server with such static DNS entries.