Edit: As Jared linked below in the discussion, this process requires that the block image file (qcow, iso, etc.) but on local storage and not being pulled over a network.
I would like to note, that "local" only means you cannot directly pull it across a network.
Using anything properly mounted works just fine. Because that is "local" as far as any commands are concerned.
I am currently migrating my home stuff from an old desktop running raw KVM on Fedora to an old HP Microserver running Proxmox 7.
I mounted the old KVM drives:mkdir /migration_a mkdir /migration_b mount -t nfs 10.254.103.5:/var/lib/libvirt/images/raid_a /migration_a/ mount -t nfs 10.254.103.5:/var/lib/libvirt/images/raid_b /migration_b/
Then ran the imports.qm importdisk 101 /migration_a/plex.qcow2 zfs-pool1 # 30GB took 9 minutes qm importdisk 102 /migration_b/nas_boot.qcow2 zfs-pool1 # 20GB took 6 minutes qm importdisk 102 /migration_b/nas_data.qcow2 zfs-pool1 # 2TB, guessing 12 hours