If you wish to take advantage of one or more of the add on packs during the Kickstarter campaign, all you need to do is click on the "manage your pledge" button at the top of the project page and increase your pledge accordingly. Once the campaign is over you will be sent a survey asking you how you wish to spend the money you have pledged.
Example: You have decided to support the game and have chosen the "Barbarian" pledge level. This means your current pledge is now $90. Afterwards you decide you wish to purchase the "Sabertooth Tiger" add on which costs $10. You click on "Manage your pledge", change the $90 that is shown to $100 and then confirm the new pledge by following the normal steps. Once the campaign is over the survey will allow you to tell us that it's the "Sabertooth Tiger" that you wish to spend the additional $10 on.
Note that you can also chose to spend this money on other add-ons or even select add-ons that surpass the price of your original pledge and pay the difference via paypal!