How do I import a second photo library into iCloud Photos?
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.