Currently the only supported workflow to add events to a json file is in Spine and then export them, sorry! Please note that in case you don't have the Spine project files, you can use Spine - Import Data... to import a json file, then add your events and re-export.
There is one exception where you can place events in Unity:
When using SkeletonMecanim you can actually add Unity events to each AnimationClip by the normal workflow of using the Animation (not Animator) window. This is not a comfortable way though, since no preview is shown. This is not an officially supported workflow and your placed events might be removed when re-exported, there is an issue ticket here dealing with keeping already placed events:
https://github.com/EsotericSoftware/spine-runtimes/issues/1838
