Good Morning,
No, you don't have to open the case to do this. When you attach the balance cable, you have 4 connection points on the end of the cable. the first two are pos and neg cell 1. Points 2 and 3 are pos and neg cell 2, and 3 and 4 are pos and neg cell 3. Use a meter to determine which one are which. I simply attached some 24 ga wire to the charger I used, set it to 4.5 volts and put the wires on the appropriate connection spots. I then monitored it with 2 meters. 1 meter in parallel to monitor voltage and I put 1 in series with the positive to monitor the current. You do not want the current to get too high. Reading the threads, it seems that the cases are hard to open. They would be glued together. If you open it, photograph the connections before disconnecting anything. You don't want to mess with a short on a LIPO battery. It could potentially cause a fire. As for an aftermarket charger, I use a ONYX 255 dual balancing charger. It doesn't put batteries into storage charge though. It is significantly faster charging than the factory charger.
Have fun with your new drone!