Hey,
I would use a Watch a Folder for new JSON files input to read the files in. If there aren't a lot of files, you could copy them by hand one at a time into the folder that is being watched. If there are a lot of them, you could set up a python script to do it at a sustainable rate.
Just be sure to copy them into the watched folder using a different extension than is being watched for and rename them once they are copied. Basically, you want to be sure all the data is written to the file in the watched folder BEFORE GeoEvent tries to open it up and read it. For example, set GeoEvent up to watch for *.json files, copy all of the data files in as *.txt files, then rename them one by one to have the *.json extension.
More details can be found in the following blog: https://community.esri.com/people/eironside-esristaff/blog/2019/04/11/geoevent-input-watch-a-folder-...
