If you click on the product picture on the main page you'll see a large version of the main picture and an Image Gallery below it. If you put your mouse on the third image there's a good closeup of the pump- you can see where the tank feed enters.
That's where your onsite water would need to connect. You'd need to do a little plumbing to secure the tank feed & keep it clean when it's not in use, but it should certainly be possible.