Yes, it is possible to change the battery. You have open the case and replace it with a new one. It's not so important that the battery is from the same producer. Important is that the battery chemistry is the same. So Li-Po battery is the right choice.
I added the external battery parallel to the original battery and has been working fine for approximately 6 month. There were some erroneous information about battery status from beginning, but now is almost fine. The curve of battery life time is still a little bit non-linear, but my reader works for hours and hours. I added external batter of 5500mAh.
The key is that internal battery gauge learning each charging cycle, so it is important to empty the battery to the end and charge it to full. The battery gauge has full priority in this case and it turns off the reader although the battery is still (full) charged. Battery is full charged at 4.20 V, nominal voltage is 3,7V and the empty threshold voltage is approximately 2.9V depends of the battery type. To decrease battery under this value is impossible and it is responsibility of the hardware battery protection. But at this reader seems that the batter gauge react using its gauge charge meter.
Battery gauge is smart and learns the current flows out/in of the cell and finally accept the added capacity.