I think I stumbled on this solution/work around by mistake while trying to get an app called Duplicate Cleaner for iPhoto to work. As the app only recognises iPhoto libraries and will only work on iPhoto libraries I was stuck. My massive Aperture library has lots of duplicates and wont scan.
So I set about trying to convert the .aplibrary file to and iPhoto Library (which interestingly doesn't have a file extension to it.
So its fairly simple.
First create a new iPhoto library ( you wont use this and will delete it later)
Now using finder, open the "info" window for the aperture lib you want to change. Easiest way is to select the library (single click) and using the short cut, Command "I"
Under name and extension, delete the ".aplibrary" and hit enter. Confirm the "remove" prompt
Now drag that file (which is now a traditional blue folder) into the newly created iPhoto library
iPhoto will promot you to "relaunch" iphoto with the new library.....viola!
Checking the "info" for the library file again you will note is now identified as an iPhoto Library.
Not sure if this will solve my scanning app problem yet but it does change an Aperture Library into an iPhoto library. Which for some is a mute point because they are all now unified.
Hope this helps.
PK