Mac users coming from iPhoto may wish to move an iPhoto Library to the new Photos app. While importing is an option when first setting up the Photos app in OS X, many users many have skipped the initial setup screens and missed that opportunity to import pictures and images into Photos from apps like Aperture and iPhoto. Fortunately, it’s very easy to add an iPhoto library into the Mac Photos app at any time.
Much like in OS X, you’ll need to use the Option key during application launch to be able to migrate an iPhoto library into Photos app.
If you already have a busy Photos library, you’re going to be working with two different image libraries now, which is why it’s generally best to allow Photos app to pull in your iPhoto or Aperture library on first launch.
If you’d like to merge two different libraries, well, at the moment, there is no way to directly perform a merging of an iPhoto library with a Photos library without manually intervening and importing photos on your own. You can do that through the File > Import menu item, or by dragging and dropping images from one library into another using the file system. Being able to directly merge libraries together would be pretty useful, so it’s possible that such a feature could be introduced into future versions of Photos for OS X. In the meantime, you can always create new libraries and use separate libraries by switching between them by using the Option key on Photos app launch.