I've managed to get OMV (usul) and Proxmox (6.) running side by side using this as a guide:
I have 4 WD Red 4TB drives in the PC, and when I installed proxmox I selected RAID10 as the installation option. This all worked fine, and it created the rpool (zpool status below). I cannot get OMV to recognize the existing pool (rpool). Is there a way I can configure this by hand? The GUI doesn't seem to want anything for ZFS other than empty disks. I'd really like to manage the datasets and snapshots on ZFS in OMV.
Of note: I was not able to install openmediavault-omvextrasorg package. I did edit openmediaault-zfs_5.0.1_amd64.deb to not list this as a dependency. It didn't seem necessary in this case (but I could be wrong).