I found myself with a little problem the other day, I needed to merge a very old aperture library with a new one, agreed that this can be done exporting and importing projects, but I'd rather look for a faster way than export import...

Here's a way:

1.- Find the library from which you will export your projects in the Finder.

2.- Option click on the icon and select the "Show Package Contents"

3.- Select all your projects that you need to extract from this library, open a new finder window and copy them inside the destination library by showing the package contents of the destination library (please note that there are no projects with the same name, rename them in the destination library as needed).

4.- After copying all the projects, open Aperture while pressing Option+Command, (If from the dock, click the aperture button and immediately press both keys), a dialog box will appear.

5.- Click on the rebuild now button, and allow the progress bar to finish.

DONE!

Happy Merging!