To force piHole to push a defined IP address as DNS server to DHCP clients:

1. Edit /etc/dnsmasq.d/02-pihole-dhcp.conf file:
   #> sudo nano /etc/dnsmasq.d/02-pihole-dhcp.conf

2. Add the IP address to be push as DNS server:
   dhcp-option=6,<Pi-holeIP>,<SecondaryDNSServerIP>

3. Save file modification.

4. Restart pihole-FTL server:
   sudo service pihole-FTL restart

⚠️ You would have to release/renew DHCP lease on each host after that to pull the new settings.
⚠️ Change is not persistant over time as this file can be overwritten by piHole.