If it has a blower to control air flow from freezer to fridge, you might be able to hook it up to the STC and have the STC control the blower. It might vary from model to model, but the freezer over fridge models I've seen don't use blowers, they just use baffles. Setting it to the coldest setting opens the baffle all the way while setting it to a warmer setting partially closes the baffle. Most people use the STC to switch power to the compressor.
If switching power to the compressor, you usually can't reliably say your freezer will stay cold enough to work as a freezer. For example, say your set temp on the STC is >= ambient temps, it's not going to switch the compressor on until the temp reading of the probe exceeds the set temp...which means your freezer compartment is warming and probably approaching equilibrium with the fridge compartment. Make sense? Say you're using it for a ferm chamber and you brewed a saison and your setpoint is 75F. Then let's say the ambient temp is 72F. It's going to be a long time between compressor cycling and the freezer compartment temps are going to climb close to 72F. On the other hand, if you're doing a lager at 52F and your ambient temps are 95F, it may cycle often enough that your freezer compartment stays close to freezer temps.
