Yes! Each Add-On has a dollar value listed next to it (most are $2). Simply add the dollar amount of the Add-On you want to your pledge. For example, if you selected the base game ($14) and wanted to add the Sunny Days Expansion to your pledge ($2) your pledge total would be $16. The Add-On details, which Add-On you want with your pledge, will be handled in the post-campaign survey.
Also, Add-Ons are 'price per game', meaning if you ordered 2 base games ($27) and wanted the Sunny Days Expansion for each game, you would add $2 per game for a total of $31.