Is it possible to modify the freezer on a mini fridge?

It’s possible to modify a mini fridge but it’d probably turn into a small R&D project.

Cooling occurs in the freezer compartment and the cold air is circulated through the fridge to cool the lower compartment, so if you were to enlarge the freezer section, the compressor will need to run longer in order to maintain temperature which will in turn reduce efficiency.China Mini Freezer factory

Taken to an extreme, if you were to remove the partition altogether, then the system will attempt to freeze the entire volume of the mini fridge in an attempt to turn it into a freezer, if it can keep up with demand.

So if you change the partition, then you’ll need to monitor the temperature of the two compartments and adjust the thermostat or change the temperature sensor location as necessary. It’s going to be an experimental process but should be quite doable if the change in volume is within reason.

Of course this assumes you’ve figured out a way to physically create a new partition.