Migrate VM Backup from 3.0 to 1.8 / How to convert vma file to raw?

Since release 2.3 Proxmox VE is using Virtual Machine Archive format (VMA)

How is it possible to backup a VM from a Proxmox VE 3.0 and restore it on a Proxmox VE 1.8?
Is there a converter to convert vma to raw/tar format?

Maybe there is only a special way to do the backup? We used
vzdump <id> -mode snapshot -dumpdir <targetdir>
and get a *.vma File.

If we use this file we will get this error:
pve:/backup# qmrestore vzdump-qemu-116-2013_07_25-12_51_47.vma 116
tar: This does not look like a tar archive
tar: Skipping to next header
tar: Error exit delayed from previous errors
ERROR: archive contaions no data

Thanks in advance for your help!

P.S:
we plan to update the pve 1.8 to 3.0, but it would be great if there would be a way to restore without updating the pve to 3.0.