Create file /etc/kresd/custom.conf with this configuration:

local forward_rule = policy.add(policy.suffix(policy.FORWARD('<DNS SERVER>'), policy.todnames({'<FORWARDED ZONE>'})))
policy.del(forward_rule.id)
table.insert(policy.rules, 1, forward_rule)

where <DNS SERVER> is address of DNS server to which we will forward zone <FORWAREDED ZONE>. As this configuration file is appended at the end of the generated configuration we have to remove the rule and then force it to first position as last configuration option in default turris configuration redirects everything to upstream DNS servers effectively causing to ignore any policy added in custom configuration.

Then edit /etc/config/resolver file and at the end of the section config resolver 'kresd add line

option include_config '/etc/kresd/config.conf'

And restart Knot DNS server by

/etc/init.d/kresd restart

Generated configuration file that is actually used is stored in temporary file /tmp/kresd.config.