De acordo com o seu FAQ , isso não é suportado:
Can I back up using FTP?
No. At this time, CrashPlan must be installed and running on the destination. We may support FTP at some point, but it would be far slower and less of a guaranteed store than running CrashPlan. With CrashPlan running at both ends, the destination software simulates restores and validates files when the destination is idle and not in use. This is a big deal - something you cannot do with FTP efficiently - and ensures that your files are in good shape when you need them