You can read in more detail about how this works at Migrating iPhoto and Aperture libraries. The migration assistant will list all the iPhoto libraries on your machine and let you migrate any number of libraries to Photos in succession. PowerPhotos can assist with this process if you select “Migrate iPhoto Libraries” from the File menu. In order to see your libraries in PowerPhotos, you will first need to migrate your libraries from iPhoto to Photos.

If you have been using iPhoto Library Manager to keep track of your iPhoto libraries, PowerPhotos can help you with your move to using the new Photos app.