As far as I know, you can't do that (have a datatore which points to a USB drive mounted to the ESX).
What you Could do is:
- have a VM which offers NFS or iSCSI services running on that ESX (a storage appliance)
- mount the USB on the ESX and connect it to that VM
- export a new NFS mountpoint / define an iSCSI LUN on the storage appliance (which points to the filesystem on the USB drive) and add it to the ESX
- create a new VM, browse the new datastore, and choose the right ISO (from the USB disk)
Another idea would be:
- instead of USB, burn a CD/DVD with the ISOs you need
- insert the disk in the CD Drive of the ESX host
- when creating the new VM, you can add a CD Drive which points to the host's CD Drive (and hence to your CD) using "Device type > Host device"