On the engineering side, the extender has quite a few complications. What is the pack voltage and how is it connected? If the extended pack is 800V, do you connect it in parallel? Will that even work with two different currents? Then you also need to split the extender into two 400v packs for charging. An alternative could be any voltage (278v for a random example) and it is boosted to ~800v via electronics to match the main pack voltage. This is costly but technically possible and makes charging easier. While some think no heating/cooling is needed, it is needed and has to be in the coolant loop. The extreme temperatures that are encountered in a vehicle require heating/cooling for longevity, no matter the battery chemistry. Now it could have a separate compressor and heat/cooling loop, but that seems way more complex and costly - but could be needed if the thermal load is beyond the existing loop.
So, IMHO, there is no way this is removable by the user. The HV is too risky to allow it, and the liquid connections have their own problems to ensure no bubbles get into the system. I also can't see users dealing with a 600 lb pack. What if a user drops it? It seems to me, strictly a one-time Tesla installation.