The answer is yes, you can pull (retrieve) data to Thingworx from an external device. You can even use the RESTful API to do that. All you have to do is create a service inside a Thing in your Thingworx environment and you'll have to use the get method.
But, as I understand your problem is not retrieving the data, but you don't know for sure what sensors you have on your device. I am not familiar with Insteon Hub so I can't be of much help on that area, but like I said you should be able to use REST API to retrieve a value from an external device using the get method.