Here is a bit of research on the idea of adding a second battery:
1. Don't try this unless you can work safely around high voltage electricity and electronics. If your idea of checking 400VDC is to use your $5 multimeter probes barehanded, you probably don't have any business with this hack. There are a dozen videos on Youtube of people hacking spare batteries into Nissan Leafs, with the connections laying about uninsulated and loose in the trunk. There is also a great video of the aftermath of a vehicle fire in a Leaf with a hacked battery. Batteries are safe if handled safely. They are bombs if handled unsafely.
2. Smart ED 2014 (451) service manual here: http://autocats.ws/manual/sdmedia/me...001_001_en.pdf
3. There is sufficient room between the rear bumper and the back of the motor/tranny/etc for a module that is 711MM(28") X 152mm(6") X 279mm(11"), theoretically if this were stuffed with 18650's this could be a 57 kg(126 lb) battery holding about 9.4 kwh, or a 50% range boost. There is some airflow through here, but given the car actually has a radiator to get rid of motor and battery heat, it is probably not critical. No reason to put the battery in the cab, except it'll get cold in the winter giving poor performance.
4. A simple, very low tech, safe, and very inefficient way to use such a battery would be to arrange it as a 36V battery, use a 36VDC to 240vAC inverter https://www.aliexpress.com/item/8000...656291241.html
and simply hook it up to the charging port in the middle of your trip. Almost all my trips involve a long stop in the middle, I'm at the grocery store or a meeting or whatever. Just have the car bootstrap itself, or stop when your range anxiety gets too hairy and plug yourself in, read a book for a while. The inverter is 80% efficient, meaning you lose a lot of juice, inverter needs to be fan cooled and cannot be underneath the vehicle. Put the inverter in the cab, maybe under the seat, and leave the windows cracked open on a hot day. 36V is a lot safer than 400V, but still if you touch two bare terminals you basically just started up a 1000A welder, so be very careful. There's your hack.
5. The next level hack, in theory: Here's a whole series of videos showing a guy hacking into a Nissan Leaf battery pack, Smart has a lot of the same general components: https://www.youtube.com/user/passat9596/videos