The problem you face is that these appliances are supposed to really be "black boxes" so anything you try to do to them is likely to be unsupported (Unfortunately not true open-source freedom)
If you need to do this, could you deploy a linux VM of your choosing and tun the cronjob from there rather than from the VDP appliance?
Sorry I can not be any more help, if this was a VDP feature then I'm sure we could resolve