PVE Adopt a Native LXD/LXC
From WikiMLT
Converting an LXD/LXC to a ProxmoxVE/LXC is pretty easy, because the are (almost) the same thing.
1. Create a backup of the LXD/LXC container. For details read the relevant section "Backup LXC Containers" from the guide "LXC Basic Setup".
2. Copy the backup of an LXD/LXC container to a ProxmoxVM's template directory within its storage. In my case I'm using a command as the shown below.
rsync --progress \
remote.host:/mnt/backups/lxc-webserver-backup-2022-09-23.tar.gz \
/mnt/pve/ssd-1TB/template/cache/