You sure can. Kickstarter will let you change the amount you pledge at any time during the campaign — just click on the button to do that (near the top of the page), and change the amount you're pledging to the appropriate amount. See our pledge calculator link partway down the Campaign page if you need help figuring out what that amount is!
If you miss the chance to increase your pledge during the campaign, don't worry; we'll give backers the chance to add more sets in BackerKit when we launch that after the campaign concludes. And the sets we'll offer in BackerKit will include our existing in-stock Fate Dice line as well as all the new items funded by this campaign.