@boros Is this possible to do, probably yes.
There are a few approaches you can do depending on how comfortable you are with syslinux.
Lets get some common terms here.
FOG uses a boot loader called iPXE. Syslinux is al alternate to iPXE. What you can do with syslinux you can do with iPXE. iPXE has two boot loaders depending on the firmware version of the target host. For bios computers the boot loader is undionly.kpxe and for uefi its snponly.efi or ipxe.efi.
So if you still pxe boot with syslinux, you will have to chain load ipxe from your fog server from syslinux.
An alternate is to pxe boot into FOG using ipxe and then call WDS from a custom fog ipxe menu and do away with syslinux since ipxe will be doing that function. I have a tutorial here on how to pxe boot into different OS installers from FOG: https://forums.fogproject.org/topic/10944/using-fog-to-pxe-boot-into-your-favorite-installer-images
The structure of a syslinux config file is similar to iPXE so its possible to translate the formats and commands between the two different boot loaders.