Go to the basic Photos view. Select all the photos you want to combine, both the new stuff and the old. Now go to File > New Album. This command is misleadingly named. When you activate it, you'll see a dialogue box that has two widgets -- a dropdown menu, which will default to saying "New Album", and a text-entry box, which is for the name of the new album; it defaults to using the location and date metadata.
However! If you click on the dropdown menu, you will see a list of all your existing albums. If you imported your Events from iPhoto, you'll see those nested under an iPhoto Events option. The events are sorted by date. Find the event you want to merge stuff into. (Note that if you initially click-and-release on the dropdown, you can navigate the menu by using the arrows or by typing the name of the thing you're looking for. So most likely what you want to do is click-and-release on the menu, arrow-down to iPhoto Events, arrow-right to get into that menu, then type the name of the event. This may be problematic if you've used very similar names a lot. Though I think you can find the first thing, arrow-down past it, then start typing again, and it always searches forward from wherever you are in the list.)
Anyways, once you've selected the correct album, the text-entry box will go away. And then you just click OK, and it merges the new and old stuff together in that album.
This is, of course, incredibly stupid. The old way was much easier, and this is a task that I'm sure millions of users want to do fairly regularly. I'm almost impressed that the Photos developers were able to get this PoS approved for distribution. :-P