I'm not clear what you are asking.
Do you want to build the core agent tools? Read the docs on
qubes-builder. LinuxFX is based on Ubuntu so you can probably use the
Ubuntu tools as is.
Do you want to build a template? Treat it as a flavor of Ubuntu, so you
build (e.g) focal+linuxfx.
If you want to create a Qubes based on LinuxFX instead of Fedora, then
that's far more difficult. You'll need to build all the components for
Ubuntu.
An alternative would be to use Fedora as dom0, and try to implement
LinuxFX as sys-gui.
Whatever your aim you should read the docs on building.