I had this idea a while ago and gave it some thought.
For anyone who is interested in this, I encourage to take a good look at the service manual available online. (search the forums if you can't find it, I think a legit copy is only 20$)
Anyways, I think it should be possible to work by placing a similar voltage pack in parallel with some considerations.
There is a service cut-off switch under a panel on the floor. This disables the Leaf-pack by breaking the connection MID-PACK.
If you were to have another pack in parallel, you would have to have another service cut off switch in the same spot as the OEM one.
A DC-DC converter should not be necessary if you have the right kind of cells, the only time there would be a huge current draw is when you are connecting cells parallel to a live pack and they are not voltage matched (this would be a stupid thing to do)
As the cells drain, the CURRENT draw will shift from one pack to the other depending on which has a higher nominal voltage. (apparent voltage will be the same when they are connected in parallel)
the other issue is the aux pack would have to dis-able itself when it is drained, but then you would also have to re-enable it at the right time when charging. You would have to know EXACTLY the behavior of the Leaf pack voltage under all conditions; Driving, charging (3 levels = 3 different voltages), and what that charge curve looks like. I would use LiFEPO (lithium Iron Phosphate) batteries, these do not have a thermal run-away and do not require cooling. (they work better when warm). but you have to make sure not to over charge or over discharge, but I would have to know the maximum voltage limits of the leaf pack charge curve (probably highest with chademo charging)
I'm sure there will be an Ingineer type product out in a year or two, the answer as to why it isn't out now should be painfully obvious, it is WAY TOO EARLY. How long did it take to develop the plug-in pack for the Prius?