if you create a VM a config file is created. see /etc/pve/qemu-server/VMID.conf
if you use local storage, the disk image file is stored under /var/lib/vz/images/VMID/ - copy the file to this location, e.g. with scp. if you are no windows, use winscp. as soon as it on the right place, unpack with "gunzip hdd.gz" and rename it.