how to merge my iPhoto Library into my Photos System Library or how to import photos from iPhoto to Photos?

Search your iPhoto library. click on it. A message will ask if you want to use the iPhoto library to your photo library. Click ok.

Photos does not support the merging of libraries. You cannot add one library to another. You can however have multiple libraries and switch between them by holding down the alt-options key while launching Photos.

if you need to merge several libraries into one larger library do that before migrating iPhoto libraries to Photos. The full version of iPhoto library Manager can merge iPhoto Libraries or Aperture. alternately export the photos from one library and import them into the other.

As léonie mentioned, you can export photos from your iPhoto library & import them into Photos. This is useful if Photos has already migrated your iPhoto library & you have either added new items to the iPhoto one after that or you need to add location info to a photo that lacks it (because currently Photos does not support adding or editing location info).

There are a couple of things to be aware of with this. If you choose iPhoto's export original or current options, no location or keyword info will be exported, as indicated by the options being greyed out in the export dialog. (Confusingly, if you choose the png option, if these options are checked, even if they are greyed out, the location info will be exported.) The TIFF & JPEG options give you the choice of including location & other metadata or not, depending on the boxes you check, but the TIFF option typically will create a file much larger than original JPEG's & the JPEG option forces you to choose among different quality levels without any indication of which one is the best compromise between small file size & preserving image detail.

Basically, you will have to use trial & error to see which option(s) work best for you, & in what situations some might be better than others.

If (like me) you think Photos should provide an alternative to make this process simple & easy, please let Apple know, using the form at https://www.apple.com/feedback/photos.html to tell them about it. Personally, I would like to see this done with better support for importing photos from the iPhoto library via the Media Browser. The Photos import function does support using the Media Browser to import (some) items from the iPhoto Library, but (among other things) it does not import any location, keyword, or similar metadata that was not embedded in the original, making it pretty much useless for this.