Adding a custom device handler is pretty simple, just follow these directions. But please note that this FAQ was created when there was only one SmartThings mobile app, which is now called “classic.” The newer ST app may or may not support a custom device handler like the one that I linked to above.
Which app are you using? And have you checked whether the built-in device handler for z-wave temp/humidity sensors has a setting for inputting a temp (or humidity) offset? It’s possivle they updated the device handler at some point, I haven’t even checked in a couple years at this point.