mggillespie,
Thanks for getting in touch! This did indeed used to be a separate plugin, but we’ve incorporated it into the Core plugin and have made some significant improvements since then. Now all of the CSV importer code is currently located in ‘the-events-calendar/lib/io/csv/’. There’s currently no supported way to customize the CSV importer, so you’d need to make modifications to the plugin directly in order to do this.
Just keep in mind that any changes you make directly to the plugin will be lost when you update the plugin, so you’ll want to keep track of any changes you make in order to re-apply them after updating.
Hopefully that points you in the right direction, but let me know if you have further questions. Thanks! π
-Casey-