not easily, you'd have to make the backup newly with the proxmox-backup-client (you should be able to include the '.img' file though) just note that its currently not possible to upload a backup with a timestamp older than the newest already on the pbs (so you'd get the 'old' backup but with a newer timestamp)
if you just want to save the backup e.g. offsite, i'd recommend either using the tape backup feature for that, or a second pbs with sync jobs (those can be synced back in a disaster recovery scenario)