Ok, the people that sold us the application took their words back: apparently, backing up to a file isn't possible with ArcServe2000... However, they think (!) that the Image Snapshot option might offer this possibility. I haven't found anything about this option on the CAI site that points towards backing up to a file , so if someone could give me some details about this separately sold plugin, it could save me the effort (and cash) to buy it too.
Kind regards,
Tim