Kevin Bowling has an article that shows how to setup a UEFI system to work with FreeBSD — including ZFS on root — and another UEFI OS like Windows.
https://www.freebsdnews.com/2017/01/23/freebsd-uefi-root-zfs-windows-dual-boot-kevin-bowling/
https://bsdmag.org/freebsd_uefi_root/
I’m not sure if this article is an improved version of or just a rebroadcast of:
http://kev009.com/wp/2016/07/freebsd-uefi-root-on-zfs-and-windows-dual-boot/