Out of interest, I looked for the LEAF battery module specs online and found this:
http://www.electricvehiclewiki.com/Battery_specs
I'm guessing that 30 modules could comfortably fit in that space, which would add 13 kWh of additional charge - not too shabby! Apparently, the 48 modules in the stock pack weigh a little over 300 lbs, so adding 30 more modules would add roughly 200 lbs. So, an after market pack that was custom built to fit that space would likely be less than 300 lbs.
The cheapest used module price I can find is $100 US though, so it would likely be $4000+ US for an aftermarket add-on pack.
At that price, it makes way more sense to buy a replacement pack from Nissan. If an aftermarket add-on pack was available for $2500, then that would be compelling for me.