DNS resolution is delegated from resolv.conf to dnsmasq:
you may create a templates-custom…
mkdir -p /etc/e-smith/templates-custom/etc/resolv.conf/
…putting desired nameserves into something like /etc/e-smith/templates-custom/etc/resolv.conf/39dnsCustomResolver
Something similar is supposedly done on template expansion when dnsmasq is not used/enabled.
AFAIK NethServer does not make use of Network Manager.