[itimpi]

I do not believe there is any way to do this unless you side-loaded them into the Kindle app (if you can even do that on an iPad) bypassing Amazon. I am assuming they were not side-loaded as if they were you would already have them available to load into Calibre.

The way to get Amazon Kindle books to a PC is to install the Kindle for PC software and then go to your Amazon account and mark them all to be downloaded to Kindle for PC. Then when you start up the Kindle For PC app they will be downloaded to the My Kindle folder in your PC's Documents folder. You can then add them to Calibre.

Before adding such books to Calibre you might want to install Apprentice Alf's Calibre plugin so that the copies in Calibre are DRM free.