Sphere colliders (and compound colliders) will come in a future update, but I'm currently concentrating on 1.80 with platform specific atlas loading.
You have all the sprite positions by accessing the spritedefinition sprite.GetCurrentSpriteDef().positions. The bounding boxes are precomputed at "Commit" time, so there isn't any runtime code that does any of this stuff, but the data is there for you to do it should you want to.
With the positions array, you can find the average position in the point cloud to find a sensible center point for your sphere, and then find max(abs(x),max(abs(y),abs(z))) from the center to find the largest radius.