There are several issues with wanting to just add to the existing battery pack. The manufacturers could certainly design the car to be capable of it, but it's probably not something you can do as an end user.
First, the car is balanced for a certain amount of weight, and the motor control algorithms are optimized for this. Adding more weight to the rear could hurt efficiency, and depending on the amount of weight, braking and handling could suffer.
But the major concern is how the batteries work. Regular 12V lead-acid car batteries are old, "dumb" technology thats easy to charge and pretty forgiving, electrically speaking. Lithium Ion batteries are the total opposite.
Lithium batteries are found in almost every type of device these days, so they seem like they are easy to use and very safe, but thats not the case. Every device with a rechargeable battery must have a dedicated charging circuit to handle the battery. It has to charge at the correct rate, stop charging when the battery is full, and also cut power to the device when the battery gets too low. The charging circuit is made for the specific battery installed in the device, and if any part of it fails, it can start a fire, or kill the battery permanently.
So, knowing that Lithium batteries are hard to maintain and require a specially designed circuit for charging, you can probably see why you can't just "add more batteries" to the car. The dedicated electronics wouldn't know about the added capacity, and the charging algorithms would be off. Parts of the battery packs have temperature sensors for safety, and you would have no where to connect an additional temp sensor.
So, what could we do? Well, if the car battery was divided into modular "packs", the car could certainly be designed to accept any number of packs. An example of this would be a lithium battery pack on a modern power drill. All the electronics to maintain and charge the battery are integrated into the pack, not the drill. That makes the batteries more expensive, but it also makes them self-sufficient and it makes it so every drill/saw/sander/radio that uses that battery pack doesn't need those complicated parts and allows them to take batteries of different sizes.