either the proxmox kernel is not used or it's a must use, to enable zfs to function from a user perspective and yours
But the proxmox kernel is not required. It makes things easier though. It is just hard to get the dependencies in the plugin package to do the right thing depending on which kernel is installed. The plugin also works on non-amd64 platforms. So, the proxmox kernel cannot be a requirement there.
The following allows installing the zfs plugin when the proxmox kernel is installed because it forces the proxmox packages to be used.
apt-get install zfsutils-linux=2.1.1-pve3 zfs-zed=2.1.1-pve3
apt-get install openmediavault-zfs