Does theme customization support changes to import code?

I successfully edited the theme using the instructions on https://theeventscalendar.com/knowledgebase/themers-guide/ (specifically to make the list view show the_content() rather than the_excerpt() and to omit the_excerpt() in the day view) but I was having trouble getting a modification to the CSV importer to work.

The code is buried much deeper (lib/io/csv/admin_views and lib/io/csv/classes) so I tried including those deep trees in my theme subdirectory, but it didn’t seem to take. Is it possible to alter the plugin’s behavior this way or would I need to modify the code directly in lib/io/csv/*?

https://www.ads-software.com/plugins/the-events-calendar/