OK, here's a start.
In Bluetoothm the PC is usually the "master" and the accessory is usually the "slave." So, you want a USB to Bluetooth slave adapter, but all the USB to Bluetooth adapters you see for sale are USB to Bluetooth master adapters.
So, here is the start to building your own:
http://www.elechouse.com/elechouse/index.php?main_page=product_info&cPath=90_91&products_id=602
It seems from the doc that you attach the USB-to-serial module to the disk, mouse, etc, then plug that into the Bluetooth Slave module and it works.
I'm going to buy one and try it for connecting my IBM Model M keyboard (modified to USB by UniComp) to my Galaxy S III.
Jamie