From HOWTO: Using pi-hole as LAN DNS server
Create a second dnsmasq configuration file /etc/dnsmasq.d/02-lan.conf
addn-hosts=/etc/pihole/lan.list
After this, create a “hosts file” for your network /etc/pihole/lan.list with the format ipaddress fqdn hostname, eg
192.168.1.40 marvin.your.lan marvin 192.168.1.41 eddie.your.lan eddie 192.168.1.42 hactar.your.lan hactar
…substituting your.lan for whatever you want your domain name to be.
On your DHCP server you will also need to set your search domain to whatever your.lan corresponds to.
Finally, restart your name server:
sudo pihole restartdns