Welcome to the Pi-hole community, @ECM56.
Pi-hole is running a tailored version of dnsmasq FTL DNS™ ( pihole-FTL ), but configuration is very much the same as with the plain vanilla version. As such, its configuration options are identical to dnsmasq's options.
There are a few caveats, however (click for details)
Since Pi-hole is also configuring the dnsmasq part at its own discretion, you have to make sure your settings won't collide with Pi-hole. Pi-hole's own settings will likely overrule yours, but they may lead to unexpected behavious as well.
Pi-hole is striving to make installation and maintenance as easy as possible for standard home users who can be only mildly computer literate.
As such, Pi-hole assumes that you are running a single subnet for your home network.
If your intended network setup comprises several subnets, you could cater for them as usual, in all ways dnsmasq (and your gateway/router/switch, of course) allows, but don't expect these settings to be supported or even reflected fully in Pi-hole's UI.
Some valid dnsmasq configuration settings may even compromise the UI in ways that'll force you to stick with editing the dnsmasq files manually, forfeiting the use of Pi-holes admin panel for lease configuration.
For that reason, I would recommend using your router's DHCP server instead of Pi-hole's for more complex network setups.
Also, while Pi-hole is using the very same configuration files as dnsmasq, the binary has changed. This means you won't be able to run dnsmasq commands or to use all of dnsmasq options on Pi-hole's replacement binary.
And finally, you should be aware that Pi-hole may alter its configuration files, e.g. based on user interaction via UI, command line (i.e. during a pihole -r reconfiguration) or updating (i.e. pihole -up).
Now that you've been properly cautioned:
As you assumed, Pi-hole's dnsmasq configuration is located in a bunch of files in /etc/dnsmasq.d/
Read more at What files does Pi-hole use? (which covers not only dnsmasq files).
Before trying to import or rather reuse your static leases:
Please make sure that DHCP is disabled on Pi-hole.
A. Prepare a static lease conf
First, you'd have to separate your static IP address assignments from the rest of your legacy dnsmasq configuration into a separate file.
You could then copy that file into Pi-hole's standard static lease file name (note that normally custom configurations should go into custom files, but here we are trying to reuse an existing config):
/etc/dnsmasq.d/04-pihole-static-dhcp.conf
You can check whether your dnsmasq configuration is syntactically correct by running
pihole-FTL dnsmasq-test
Sort out errrors until this results in syntax check OK.
B. Check Pi-hole's UI for possible misalignments
On a client machine, open a browser and go to Pi-hole's DHCP panel under Settings | DHCP.
Even though DHCP is disabled, Pi-hole will populate the Static DHCP leases configuration section in the lower half of the screen from its 04-pihole-static-dhcp.conf.
Check that the lines there make sense, i.e. an entry should match its heading. If things are mixed up, your configuration contains extra parameters that are not handled by Pi-hole's UI.
C. Enable DHCP
If everything in B. looks ok, you are almost done.
On Pi-hole's DHCP panel, configure and enable DHCP and press Save to restart Pi-Hole as your DHCP server with your existing static leases.
However, if you encounter misalignments in step B, you have two choices:
1. Recommended: Edit `04-pihole-static-dhcp.conf` to get rid of your extra settings.
Make your lines conform to
dhcp-host=MA:CA:DD:RE:SS,192.168.1.100,laptop
where you replace MAC, IP and name with your correct settings, of course.
This will allow you to edit static leases via Pi-holes DHCP panel further on.
When finished editing, resume with step B.
2. If you are desperate for your extra settings, you can try and keep them.
BUT: You should refrain from using Pi-hole's DHCP panel UI in the future, and take extra precaution when enabling or disabling DHCP.
Before starting DHCP, you have to move 04-pihole-static-dhcp.conf to another location, e.g. your home directory. Otherwise, if you'd enable DHCP and Save those settings, your static lease file may get corrupted.
So move it out of the way, then configure DHCP, enable it and press Save.
Copy 04-pihole-static-dhcp.conf back into /etc/dnsmasq.d/, but this time, use a different name like 04-static-lease-migration.conf and restart Pi-hole's DNS:
sudo pihole restartdns
Verify with:
sudo pihole status
If this doesn't show you the green lights, you have introduced conflicts in dnsmasq's configuration that you have to deal with.
If everything else fails, move or delete 04-static-lease-migration.conf and restart Pi-hole.
In that case, rethink if you really need those extra options or whether your router would be better equipped to handle your DCHP configuration.