If a device shows a device type Handler of “placeholder“ that means it is being managed through an integration. It will not work with any other DTH and you should not try to change it.
If the door itself shows up in smartthings, just use an automation to have the door opening trigger a virtual Sensor, and then you can use the virtual Sensor in STHM.
You can use webcore to do this or you can use the virtual device type that people use for triggering echo routines. That one is both a Switch and a Sensor. When you turn the switch on in any fashion, including with an automation, smartthings will think that the sensor has opened. When you turn it off, it will think that it closed.
Here is the code for the virtual device: