If your mini fridge has a compressor, you should be able to convert your mini fridge to a freezer by simply connecting a new thermostat to it. However, opinions are divided on how long the freezer will last.
If your mini fridge has a compressor, it usually has a freezer compartment as well. This means that the compressor can maintain freezing temperatures. Researching the type of compressor your mini fridge has should tell you if it’s rated to generate freezing temperatures for only a certain number of cubic inches or throughout the entire food compartment.
However, not all mini fridges are equipped with compressors. Cheaper models of mini refrigerators are often built with Peltier coolers, which use electric currents to cool the inside of the unit. Besides being less efficient than models with compressors, these mini fridges can only cool to about 40 degrees Fahrenheit, meaning that they can’t be converted to freezers.
If you’re planning to do a conversion, check out some engineering and HVAC forums such as HVACProforums.com and Xtremesystems.org. Current opinion is divided between those who say that the compressor will burn out rapidly and those who say that the freezer will run for a long while.