yes, you can edit photos imported with the CCK using a number of 3rd party apps.
when importing using the CCK, the full-size original photos (whether JPG or RAW) are stored on the iPad and can later be uploaded to your computer at full original resolution.
However, on the iPad itself in the Photo album, an "optimized" version (about 2048 x something resolution) is made available for viewing and accessing on the iPad
AFAIK, any image editing app which accesses the iPad's Imported Photos album can only access these optimized/reduced versions. good enough for a quick edit then emailing or uploading somewhere. I don't use Photogene that another poster mentioned, but I have the CCK and have used a couple other apps I use for this purpose.
but as I said your full size originals still exist on the iPad and can be later retrieved on your computer