Yes, you can add more than one reward in simple way. You must add new reward to your original EXISTING reward that you pledged for. You can add as many of them as you like to your current pledge. To add an add-on, simply include the amount in the pledge.
Example: If you made a $108 ($99+$9 shipping) Early Bird pledge (Phonster Full) and you want to add the one more Phonster One , simply, add $68 ($59+$9 shipping)( Early bird) to your pledge and enter "$176" without changing the reward category.
After it we will check your pledge , and after campaign ends, in survey you will write your both rewards.
In addition send us message via Kickstarter or leave a comment and we will be aware of this.