mergerfs or similar on a Mac for combining multiple drives?
The README.md file for the mergefs project on GitHub answers this for you...


  mergerfs is a union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is similar to mhddfs, unionfs, and aufs.


Emphasis Mine

Unionfs is a FUSE project (available on GitHub) that claims to have Mac support.  There hasn’t been much development in the last few years so, how well it works with your version of macOS will be up in the air.  

That said, it’s probably a better idea to just standardize the formats over those multiple drives to reduce complexity and ensure reliability.  If these drives are running in different machines with different OSes, NFS might be the direction you want to go.