Yes the file format at least is here
https://github.com/fritzing/fritzing-app/wiki/2.1-Part-file-format
as far as I know the parameters that Fritzing understands are not documented outside the source code though. If you are familiar with image processing it may go much easier for you, I haven't before Fritizing played with images. What you need to do for the 7812 is pretty simple, all you need is to edit the schematic and breadboard svg files and change the text from 7805 to 7812 and you should be away or just do what Old_Grey just suggested 
.