I have no idea how difficult it would be to do this, but my idea is to have small batteries that can plug into the laptop, like the ports, and the computer would receive charge from those when off power, while (maybe) charging them when on power. Similar to the storage expansion cards you can buy from the store.
Hate to be negative, but there’s not a lot of volume in the slots so they wouldn’t be able to add any meaningful battery capacity, particularly with the required circuitry packed in there too.
I recall seeing someone talk about possibly designing a slot-on battery pack which sits under the laptop using the slot to clip into, but I can imagine that having all sorts of weight/tension issues with the slots, and would probably require mechanical fixing to both left and right of the laptop to be steady.
At which point, honestly, a standard USB battery pack and Type-C cable would be better suited and usable for other uses too…