Welcome to the Pi-hole community, Rapsey. 
I am not aware if and how defining leases in /etc/ethers would work.
(That doesn't mean it couldn't, I am just not aware of it.)
What I can tell you is this:
Pi-hole's UI is storing DHCP lease assignments in /etc/dnsmasq.d/04-pihole-static-dhcp.conf, see also What files does Pi-hole use?.
If you have lots of assignments that require special parameters, you might have to forfeit using Pi-hole's UI.
Instead, consider creating a separate file altogether, in order to avoid UI conflicts and potential side effects with Pi-hole updates.
Newb Q: How to pull DNSMASQ config into Pi Hole for DHCP reservations may also be relevant to you in that case.