Hi!
The hardware on voxl only supports a stereo pair out of the J3 connector. However, you could absolutely plug in fisheye cameras instead of the pinhole ones if you're just trying to get a better field of view. Additionally, the system is setup to be able to support that same ov7251 camera in both the J2 and J4 ports, so you could in theory have 4 fisheye cameras concurrently.
NOTE: voxl-camera-server (opensource) is not set up to to handle multiple cameras of the same type since we don't actively support any camera configurations like that. It's very doable though, you'd have to check the camera IDs for the configuration that you create and mod camera server if you're trying to use our standard software stack (so you can out of the box support 3 fisheye cameras, and it would only take some userspace coding to support 4)