How much did you pay? Did you get doors? You can post a photo in here:
The main unofficial accessory for the Twizy that lots of people get is @kennethnilsen69 Powerbox as you've seen. Other than that, the only real accessories are things like mud flaps, bluetooth audio kits and things like that. The Twizy is pretty bare bones, but all the better for it.
The yellow one you've seen is most probably the F1 Twizy which was made by Renault as a proof of concept, it isn't for sale. However someone did post a video recently of a Twizy owner that achieved the same look. It isn't something you can buy as a kit though.
As for spare battery, this isn't some duracells we're talking about. The battery is underneath the seat and is what accounts for most of the weight of the Twizy. Not something you can plonk on the back seat unfortunately. So no spare battery no.
However, there are tons of clever people about and some have built their own batteries. If you're that way inclined I'm sure there are some people on the forum that could help you out with that.