Z3D format comes from a tool called Zmodeler and represents 3D graphics created using this tool. And it is only this software that can be used for making new .z3d files based on other content, for example from the FBX exchange files.
We do not know about any alternatives that would be otherwise capable of fbx to z3d conversion, so using Zmodeler is your only option. The software has large online community, so if you have some particular issues or trouble about how to convert fbx to z3d, perhaps it can be of help.
Updated: January 3, 2024