You have a few options:
(1) make a new management group and assign those clients that you don't want to use Pi-hole to this group. Apply no blocking to that group. The clients will send their DNS queries to Pi-hole but there will be no blocking applied and the queries will pass through to the upstream resolver.
(2) Either manually or via DHCP settings (if you use Pi-hole for DHCP) assign a different DNS server to clients: