one way to do it would be if you have a directory storage (for example the default local storage /var/lib/vz/), you could place the raw image file in the directory of your container, and add the new disk to the container configuration as a mountpoint.
for example:
Code:
root@pve:~# cd /var/lib/vz/images/131/ # replace 131 with your container ID
root@pve:/var/lib/vz/images/131# ls
vm-131-disk-0.raw
root@pve:/var/lib/vz/images/131# cp /path/to/your/file.raw vm-131-disk-1.raw
root@pve:/var/lib/vz/images/131# nano /etc/pve/lxc/131.conf
and add a mountpoint line to your container configuration.