Your setup looks OK. You don't need any special/manual settings, the addnhosts entry will be done automatically by adblock (check the file /etc/config/dhcp).
One major drawback of the 'null' blocking method is, that it only works per single host entry ... you can't block a whole domain (e.g. including all subdomains) with one single entry. That's may one reason why these pihole guys working with such big blocking lists.