To add a user property in code, simply assign a value to the property and save/update the user object. When Backendless processes the API request to create/save user object, it will determine that a new property has been added. The property will be created for all existing users. The user which is being created/updated will have the property with the value set in the code.
Important: this applies only to the apps which have Enable Dynamic User Definition option set to ON. That option is available in Backendless console at Users > User Properties > Enable Dynamic User Definition.