Is it possible to run OpenZFS on MacOS Sierra on a Mac Pro
Yes it is: I am using zfs, including after upgrading to High Sierra and after updating zfs a few times.

After updating macOS I vaguely recall reinstalling zfs for this version of macOS (and possibly uninstalling it first--fairly certain I followed the directions associated with the installer or in the wiki), and then later as zfs was updated installed the newer release. I was prompted to upgrade the zpool in the normal course of using my external drive (when I did sudo zpool import -a -f).