Here is one possible solution - http://www.zdnet.com/blog/soho-networking/turn-external-hard-drives-into-network-storage-via-usb-with-addonics-nas-adapter/322
but you will have to purchase one for each drive.
You might consider this one as well - http://www.amazon.com/Cirago-NUS1000-Network-Storage-Link/dp/B0037NYKW6http://cirago.com/wordpress/products/networking/nus1000/
There are others out there - http://www.pcmag.com/article2/0,2817,2392221,00.asp
You could also consider an NAS drive enclosure but that would require you to remove the drives from the external enclosures and is bit more expensive than the other options.