I don't own a Qualisports, but we do like our folders and there's not much space on them for batteries. So I have the ability to wire in extra packs in parallel when needed on my bikes.
You have to consider the need to remove the battery, the required waterproofing (under the bike for you) , and strain relief, If you have the solderingf skills, tap in a plug in parallel that fits your extra battery.
For a better method, you would find both ends of the aviator plug and make an auxialiary extender cable with a extra plug. That preserves the wiring on your original battery.
There's another approach that many people think is crazy,
originally written up by Micah Toll, who runs the Electrek ebike blog. He connects his spare battery's output to the charging port of the original battery, Skip the part in the video about building and see how he connects it. I was dubious, but I tried it and my spare pack did apply its power to charging the main battery. While the wiring is easier with his approach, I prefer though to run packs in parallel. It allows the load to be shared between the packs.
As in any situation with hooking two batteries together in parallel or using one as a charger, they must be at the same voltage,
