Kickstarter does not provide any kind of item selection outside of the pledge levels. However, we use a third-party service called Pledge Manager that lets us handle this once the Kickstarter closes.
There are two ways you can buy add-ons:
1) Increase your pledge by the cost of the add-ons now, and then later when the Pledge Manager survey comes up you'll be able to select the ones that you want using the funds that you've already added.
2) Just select a reward level now and don't worry about it. When the Pledge Manager survey goes out, you'll have an opportunity to add more funds for any add-ons you want to purchase.
You can even do a mix of these two - add funds now for something you know you want, and then again later if you decide you also want something else.