Buying a new NAS drive to plug in and use a portable HDD seems like a bit of an overkill to me and to be honest if I where forced to do this then I wouldn't use the portable at all.
SPR's suggestion of buying a Router with a facility to plug in storage sound like an option. My current Talk Talk Router could do with replacement and upgrading anyway.
I've been searching the web and come up with these: Addonics-NAS-Network-Adapter
Not very good reviews but seems like an affordable option, anyone had experience on this type of thing?