How do I set value of the control that is not mapped to any field?

To do that you need to return the object of the following format from the handler function:

return {
    app: {
        form: {
            data: {
                modified: {
                    controlName: value
                }
            }
        }
    }
};

Or you can do that through the API method:

DWKitApp.API.setDataField("controlName", value);

where controlName is the name of the control and value is its value.