Just one comment to add to this. Bearing in mind that I have not actually taken apart an Aspire, but I have had a lot of experience building large high voltage battery packs out of LiPo and LiFe cells for electric bikes and electric motorcycles, there's one thing I would like to point out, but it needs a bit of background first.
All lithium cells have something called a battery management system, or a BMS. Since Lithium cells are usually only 3.7 or 3.8v, you need around 80 cells to make the 300v battery that we have in our cars. The problem with 80 cells though, is that each cell might be just that tiny bit different from each other, and over a long enough life time, you could have one cell charged, while the cell next to it is close to flat. Why is that a problem? Well, overcharge a lithium cell, and you're asking for a battery fire...
So, in order to keep each cell at the same voltage, the battery management system will either charge up only the low cells (bottom balancing), drain off the full cells (top balancing), or some really exotic ones will just take energy from the highest cell and send them to the lowest cell at any voltage. The BMS also does a few other things like restrict charge rate, and restrict drain rate, but anyway.
The only reason I tell you all this, is that it is extremely foolish to run such a high voltage battery without a battery management system, and if you don't know the exact design, you'd be foolish to try to add capacity behind the existing BMS.
However, at some point, your battery stops being seen as 80 individually managed cells, and it just becomes a voltage on a wire. If you could find that point, and tap into it with your own 300v battery that has a BMS that you have the specs for, you could parallel the battery there, and let each BMS sort out its own battery.
Let's say you could buy one of these: There's a nice 30kwh pack you could use to quadruple the range we have... http://www.alibaba.com/product-detail/lifepo4-300v-100ah-batteries-for-EV_60248004058.html
That has its own BMS, that will manage the pack for you. You will almost certainly confuse the onboard computer though, as it will measure every KWh going in and out, and since you've changed the pack capacity, it might actually not use the extra capacity and indeed could even cause a fault "I've put 13kwh into this battery and it's still not full??!! Something is wrong, turn on a Malfunction Light!" etc...
So the short answer is still the same as everyone else's. Not worth the effort to try this. Long answer - there's several ways to skin a cat, and it's not by any means impossible.
One day when this car is out of warranty, if the battery pack is the first (and only thing) to break, I might build my own battery pack and try it. There's not a snow ball's chance in hell I'm playing around with it while it's still under warranty or still running well though.
