The crop ratio 2:3 is not supported at this time, and the crop lock list is not editable. While 2:3 understandably scales well to print photos (eg., 4 x 6), the ratios 3:4 and 4:3 offered in Photoshop Express for iOS reflect the aspect ratio most common to digital cameras.
If you upload photos to your Photoshop.com account using our mobile app, and then access the photos in a desktop web browser, our online editor does offer 4 x 6 as a preset crop ratio. Edited versions of your photos will then be viewable and shareable back in the mobile app (after tapping the 'refresh' button).
Thanks for taking the time to post your feedback -- such feedback helps us understand user needs.