If you create the pool on the mac... which is at a lower zfs version. Freenas will be able to use it. The latest feature flags on zfs haven't made it to Mac yet. Or you could load an older version of freenas to build that pool, where the flags match. That would allow use of the gui for creation, which is preferred. Using an older pool version on a newer freenas is not a risk. Some maintain their pool at v28 for more compatibility.