Unless there exists a known modification for a specific car model, the answer is
Practically not possible. With intricate electrical surgery, probably yes.
Modern electric vehicles manage their batteries carefully, tracking voltage per cell, current per cell, temperature, and probably cell health over time. The logic is proprietary, and can vary considerably across manufacturers.
The battery chemistry used in e-vehicles is lithium-ion. The nominal voltage of a single lithium ion pair is about 3.7V. The cars operate the motors at several hundred volts. This means that the batteries work in series, in some configuration.
The specific connection pattern can vary, and be adaptive. but will not be just a plain series chain connection, since the batteries cannot be charged in series. As the cells deplete, some do so a little bit faster than the others. Series connection enforces the same current to all cells, which would over time drive the cells into imbalance, and destroy some of the cells by overdischarge.
This is solved by "balance charging" which most people know from rc hobbies. The cells are charged individually. Similar things happen in electric vehicles.
As you cannot replicate the circuitry or modify the charge software, your best bet would be to take many many individual new lithium ion cells, and connect them in parallel with the existing cells, doing this to almost all of them.
The only thing the car can notice, is that the cells can suddenly hold more charge.
Then the question is: The manufacturer knows how big cells they used. Did they code the manager to take this into account? It could be that it will refuse to charge more than an amount that the original cells could theoretically take.
Tracking the amount of charge taken out and pushed in in relation to cell voltage will diverge from how the original cell would behave. Will the battery manager be ok with this?