We've now released preliminary mod support for Idol Manager! You can download the editor here. The editor can be used to create new events and substories, and you can also add new idol portraits.
For a basic explanation of how to use the editor, refer to this document.
To re-iterate what we've said previously during the Kickstarter campaign, support for mods ranks behind working on the core game on our list of priorities: we believe that working on core gameplay features and content is our highest priority. Certain modding features will be released after we've finished working on the game. This editor is a tool we originally created for internal use, and we wanted to give players a chance to tinker with the development tools we've already created while the game is still in beta. As such, feature requests for modding tools may not be addressed until after the game is released. (You're still welcome to share feedback, however!)
Questions? Feel free to ask here!