Not sure of exactly on what you are asking. but...
The magic photo's are created when the photographer takes your photo. They get you to pose and the image is added, at that time, to your photo. If you have already had the photo taken and they have not added magic photo, then I am not sure that you can add it after the fact.
But.. if you want to get the photopass for a future trip, then by all means ask for the magic photo and it can be added onto your photos that you can download. Order it 14 days in advance and they give you a discount.
With the Photopass, you can edit the photos that have been taken and add borders (on the website), and then when you are finished you get a download from the website, or you can order the CD to be sent to you. We get one every trip, and usually get between 100 and 150 good photos. Plus my DW always puts borders around all the photos, so we end up with around 500 downloaded each visit.
Downloads are super easy. You access the PhotoPass site, enter in the coupon code (inside of your photopass package) and then you have 30 days to view, edit, and then download all your photos. Once you do all your work, you select download from the site, and the generate a number of zipped files with all your photo enclosed. You also get a number of professional Disney park photos included.