All you need to do is increase your pledge amount (keep your rewards selection the same) by whatever amount of optional add-ons you would like to include with your backer rewards. This is done during the actual campaign itself as it helps achieve more stretch goals for everyone.
We then sort out what optional add-ons you wanted via our survey after the campaign ends. There will be yes/no questions next to each optional add-on choice in the survey. You then will select whatever optional add-ons you paid for during the campaign during the survey process.