I must say I'm a bit surprised about it, but there doesn't seems to be a way to do this. I had to try myself, but there is no way I can add the USB memory as a local storage in ESXi, hence no way of assigning it as backup storage in ghettoVCB. Unless someone has a workaround for this or want to correct me?
Virtually Ghetto to the rescue:
Beware though that ESXi doesn't like the concept of "external USB drives" especially the removable part, so unmount carefully before disconnecting.
It is not a recommended way of taking care of your backups this way so do heed Williams warning and think of a different way to handle this if you can.
Wil| Author of Vimalin. The virtual machine Backup app for VMware Desktop Products
| Vimalin : Automated backups for VMware Fusion and VMware Workstation Professional
| More info at https://www.vimalin.com
| Twitter @wilva
| VMware Wiki at http://www.vi-toolkit.com