First of all, ask your vet!!! I'm not a vet, just a random person on the internet.
Not sure what type of insulin you're using but most pens used in humance medicine are 3ml (100IU/ml, there are exceptions of course). That might be a better option. In fact our vet patients use your regular humane Lantus, Tresiba and Insulatard.
I would side on the cautious side of things and not give my cat expired insulin. I know some people do it and they're just fine but I wouldn't risk it.