It's very easy. Click the "Manage Pledge" and then change the DOLLAR amount and add in the total dollar amount.
For example, let's say you picked the "Main Even ($20)" tier and are currently pledged at $20, but you also want a T-shirt and a hoodie. Tshirts are $19 each and a hoodie is $28 each, but with the combo deal, they're $45 (36 t-shirt combo +9 for the upgraded Hoodie).
Now, click the "manage pledge" button and then change the DOLLAR amount from $20 to $65 (20 for the book+45 for both tshirt and hoodie).
And that's it! Once the campaign ends, I will send out a survey and that is where you'll tell me what items you added on (and let me know what size/kind/etc you wanted).