Well, I have just received set of cheap phone lenses and got some experience. Photos follow (nothing to write home about), but
Tello was flying wobbly and before I managed to recalibrate center of gravity, it crashed into a wall and broke propeller

So I made two photos with/without added fisheye lens while
Tello was sitting on a desk; I attach them both to compare FOV.
For what I see,
Tello has a fixed-focus cam, so it was not able to adapt to additional lens. Maybe if I made some kind of tubus I'd be able to find working position.
Without additional lens:
With it:
