I never tested it but it should work like this:
Create custom template dir:
mkdir -p /etc/e-smith/templates-custom/etc/dnsmasq.conf
Create /etc/e-smith/templates-custom/etc/dnsmasq.conf/91hosts
with following (example) content.
Just create dhcp-host entries (check MAC addresses) as you like and set the wanted DNS servers with the last option.
# host A
dhcp-host=00:15:99:27:f8:98,set:specialhosts,192.168.1.20
# host B
dhcp-host=00:51:99:72:8f:89,set:specialhosts,192.168.1.21
# set dns servers for specialhosts to Neth and google.
dhcp-option=tag:specialhosts,option:dns-server,192.168.1.254,8.8.8.8
Apply config:
signal-event nethserver-dnsmasq-update