I found this out today and tested it and it works. Inital access is slow, but once connected the access seems reasonably quick.
You can actually add anything that's USB to port A of the BT Home Hub, be it a real usb drive, camera, mp3 player, usb mem stick, or whatever has storage on it.
Attach your unit to the USB A port, then open "My network places", click "add network place", add network place wizard will open,
click next > next > browse.
This will open "Browse For Folder"
click + "Entire Network" + "Microsoft Windows Network" + "Bt" + "Thomson" and click BT_7G and click ok
This will take you back to Add network place wizard.
In this you will see \\Thomson\BT_7G. Click 'Next'.
On the next window you will see "BT_7G on Samba Server (Thomson)". You can delete this and call it something relevant.
Click next, untick "open this network place" and click 'Finish'.
Now go to My network places, and you should see a folder with the name you specified.
This drive can now be used by all network machines.
The Samba Server on the BT Home Hub uses the name "BT" as its Workgroup name, if you try to add a USB storage device to a PC on an existing network with a workgroup name other than BT you may find the BT workgroup is not available from the browse option, but if you temporarily change each PC to join the BT workgroup then it will be available from the browse option , once the device is added you can revert to your workgroup name"
Please note that all Drives connected have to be formatted as Fat16 or Fat3. NTFS formatted drives are not recognised.