I very much doubt you can use 3/32. I have checked in this diff, but did once look into it for the touring car diffs and it wasn't possible then - and there doesn't feel like there is enough side movement to allow for a smaller ball.
2.5mm isn't hugely odd - and it's a Schumacher part. As well as some other aftermarket diff ball sellers that i've seen.
__________________
Matthew White
|