Most blue tooth speakers, and most devices in general, are designed to fit as much as possible inside of the devices outer shell, that way it has the most capability possible while keeping the production costs down. I would think it unlikely that the speaker would have much room to allow for a external passive radiator to be installed. However I know some desktop speakers leave room in the form of bass ducts to improve bass, in my experience blue tooth speakers are different. There is always a chance there is room, but it would be hard to know without opening one up and looking at the internals.