Absolutely! In fact you can add any pledge level onto an existing pledge. Several people have asked this so they can have two sets of each Tribe. One pre-painted in traditional colour schemes and the second set for them to create their own colour schemes. Then mirror matches can be played as well.
To do this, simply add the cost of whatever pledge level you would like to add to your total and you can tell us what that extra is for in the pledge manager after the campaign.