You can create a virtual sensor using the official web interface to your account
However, it’s unlikely that you will be able to use that kind of sensor to trigger an Alexa routine because although the sensor will be recognized, you won’t have any way to make it change state from open to closed. The kind of sensor that allows you to do that requires that you also have a SmartThings/aeotec hub.
However, the good news is that instead you can create a virtual lock through the web interface, and you will be able to lock and unlock that with a smartthings routine, and you can use that to trigger an Alexa routine.
See the community FAQ for details and more options.
FAQ: Creating Virtual Devices with the new architecture (Without the IDE) (2023)