This mentions “prepared file” 6 times but does not explain what that file should contain: https://firewalld.org/documentation/man-pages/firewall-cmd.html
Is that an XML file just like ones found under /usr/lib/firewalld and /etc/firewalld?
Looking at that doc it seems the "prepared file’ is one the user has created to be read by the firewall-cmd and update the system accordingly. I would guess it has a specific format for each type of use but have never used one so cannot be 100% certain of the usage.
There is a lot of info about the firewall-cmd and its functions within that doc. I find nothing specific to tell the format expected for the “prepared file” is references, but they may be simply a structure similar to that received when you do something like
$ firewall-cmd --get-zones
FedoraServer FedoraWorkstation block dmz drop external home internal libvirt nm-shared public trusted work
$ firewall-cmd --info-zone FedoraWorkstation
The second command returns a list of attributes and values and it may be a similar structure for the “prepared file” referenced.
I wonder how it could make more sense to edit another file format instead of directly editing XML files of known and documented structure. At least, there is tooling for editing XML.
Do you have the known and documented file structure for those files? If so then use it. If not then it does not seem “known and documented” anywhere within this discussion.