Vmware is a great system and very powerful for any business needs. What you are attempting to do is not impossible but you need to do a few things first.
If I have understood your question correctly then all you want to do is use the USB disk drive as a datastore?
I will assume that is correct. So I will go on:
First of all try not to think of your disk as a physical disk just yet. It can all be achieved with the use of FreeNAS. You will first need to boot up FreeNAS box as a VM then make sure you format your USB as FAT32. Once you have done that I urge you to follow this guide here:
http://mikebeach.org/2013/02/27/mounting-a-usb-fat32-formatted-disk-in-freenas-8/
Once you have followed that guide, in all intense and purposes you have an external storage device. Now in order to get it to operate as a datastore you will have to create an NFS share on the newly attached volume in the FreeNAS box. Once you have done that attached it to your VM Host as network attached storage. In order to preserve your setup I recommend setting up regular backups of your FreeNAS configuration and of course your VMware config.
So if everything goes down or offline, you still have a FreeNAS volume intact on the external USB drive.
Hope that helps.
And just for the record. Its not a fix that is needed for VMWare to support attaching USB drives to the host, its a feature. If VMware decide that it is a popular feature then I guess they may include it in future releases. But don't hold out because the purpose of VMware is to provide businesses a way to harness virtualisation without the need to spend money on lots of bare metal boxes. And from a business stand point it's not a feasible feature to have USB drives lying around with maybe business critical data on it.