If you want to create custom import files (not Historical, but single use imports for Free Agency, Drafts, League Creation, etc) the easiest way to see the format is to create a league and then go to Search, Active Players, and click Export. That will put a .json file in your Documents\Wolverine Studios\DDSPF 2018\Leagues\*leaguename* folder that will show you the correct format for an import.

I can only export when I select all players...and even then, it creates a JSON file which I don't know how to read.

A .json file is a text file. Notepad, Notepad++ are tools that you can use to open it and modify items.