I have discovered for anybody else wondering that I can drag and drop new photos into already created events however I cannot create new events only new albums.
There are no events in Photos. What you are seeing in the folder "iPhoto Events" are also albums - a complete fake.
The "Moments" have replaced the events. The Moments are now the basic entities of the library, just like the events are in iPhoto. Each photo and all its versions will appear in one "Moment"; a photo cannot be in more than one moment. But you do not create or manage the moments on your own. Photos will partition the library automatically into moments, based on similar capture dates and similar locations. There is no way to move photos between moments other than changing the date of the photo. And you cannot name the moments, even the names are created automatically. To have more descriptive moments it will help to ensure, that all photos have GPS tags, so Photos can use the location information to name the moments. You have to add the GPS before importing into Photos, because Photos does not (yet) support the editing of GPS tags.
You are seeing a folder "iPhoto Events" in the Album section of the library, because Photos created an album for each event it migrated to Photos. This way the name of your previous events could be saved, and the photos are grouped together in an album. The fake events album are only created, when you migrate a photo library to Photos. For new imports you will have to create albums on your own, if you want them.
I have to say I hate the new "photo" app.
Then why use Photos? It is not compulsory.
You could continue to use iPhoto for the time being, until Apple has released the first updates with hopefully more features. iPhoto 9.6.1 is fully compatible with MacOS X 10.10.3.
The Yosemite update let iPhoto installed, you just need to drag iPhoto back to the Dock from your Applications folder. When you launch it you will see a warning, that the library has been migrated to Photos - confirm that you want to open iPhoto and not Photos.
If you not yet did update to version 9.6.1, see this user tip: Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities