You can! Pixy ships with a 75-degree field of view lens (2.8mm focal length, F2.0 aperture), which is great for robotics applications, but your application may need something different, perhaps something that can see distant objects better (longer focal length) or see more objects (wider field of view and shorter focal length). Or a lens without an IR-cut filter, so that Pixy can sense infrared.
Pixy uses a standard M12 lens. These are available through several vendors. Here's an example (There are others.) You need to choose a lens that is designed for a 1/4“ sensor or larger (Pixy uses a 1/4” sensor).
Important: when choosing a new lens, it should have an “IR cut” filter or the images that Pixy processes will have poor color response.
If you think you've found a good alternate lens for Pixy, tell us!