Everytime the DHCP daemon is restarted, the settings will be created again from scratch. Therefore you will be limited to the options you are able to create on the GUI.
In my case, I also needed to add some stuff (lines) to dhcpd.conf - I solved the first step by copying the current dhcpd.conf to dhcpd.override, make my manual changes and add some start options to use the dhcpd.override instead.
Disadvantage: if you want to change something you have to manually edit dhcpd.override each time! Otherwise you may have to use sed or similar stuff to “dynamically inject” additional options?
If you think this might be useful I can PM the link I used...