Not easily, if you want to upgrade to a larger pack several years down the road that may be possible. Aptera has to design a completly new pack for every size pack they put into the car. That’s why they will be selling the 400 mile first as it is the right balence of profit and eas of design.
I can’t imagine it would be cost effective given that the battery pack is by far the most expensive component in the car. Certainly the best option for a new pack would be to trade the car in on a newer model. As for used packs, Nissan Leafs have been upgrade to bigger packs when their batteries were replaced, but that’s always with a pack from a wreaked Leaf. The cost of a new battery pack for a Leaf is more than the car is worth.
Nathan Armstrong talked in a youtube video a bit about the ability to repair and maintain your own Aptera, and he did raise warnings about the battery and the danger of high voltage and current. You probably shouldn’t mess with the battery without the proper training and certification.