Take a look at this thread that track how we solved this issue + moving sphere:
This is how we clamp to the sphere, our radius is 5.0 (hence the magic number).
protected override Vector3 ClampToNavmesh(Vector3 position, out bool positionChanged)
{
// This code clamps the AI to the sphere of radius 5.0f
positionChanged = false;
// Normalize position then scale to radius of sphere
Vector3 newPosition = Vector3.Normalize(position) * 5.0f;
if (newPosition != position) positionChanged = true; // not sure what this does
//Debug.Log(position.ToString());
return newPosition;
}