How do I import a second photo library into iCloud Photos? Since my MacBook doesn't have enough disk space for all my photos, I took an external USB HDD and created a second photo library. That was back then with Aperture. Now I have moved to the new Photos app and the iCloud photo library. 

How do I move the pictures from the external HDD library to iCloud Photos? The Photos app refuses to use iCloud Photos when switching to the external library:
I have two solutions now. First, there seems to be some software out there, namely Powerphotos, which can manage multiple libraries for the Photos app.

Second, one can select the external library as the system library and then sync it with iCloud. This is obviously a bit stupid if you want to keep the libraries separate. But in my case this is ok, since I merely want to get all my pictures into the iCloud Photos. 

If you still need multiple libraries, you probably should use PowerPhotos and copy pictures from the dedicated libraries to the system library using that tool.