See this support article: Aperture 3.3: Using a unified photo library with iPhoto and Aperture
It will tell the features that may differ when opening an Aperture library in iPhoto. As far as images and edits are concerned, both are fully compatible. Print products and Slideshows are not fully compatible.
You are using Aperture 3.3.1, am I right? Your signature says Mac OS X 10.6.7 😕?
Do you want to combine several libraries into one big library? Since you cannot import your Aperture Library into your iPhoto Library, you can use Aperture to create a unified library by merging your iPhoto Library and your Aperture Library.
Then use iPhoto to work with this merged, unified library.
This support article tells you how to merge libraries using Aperture:
Aperture 3.3: How to use Aperture to merge iPhoto libraries
To be able to merge your iPhoto Library into the Aperture Library, you have to upgrade the iPhoto LIbrary by opening it at least once in iPhoto 9.3, and your aperture Library need to be version 3.3.1.
Make a backup of your libraries before merging, and it is a advisable to rebuild the libraries before merging, to avoid glitches caused by minor library corruptions.
Feel free to post back, if you have more questions.
Regards
Léonie