Creating inherited core files

I am looking to edit the extras.php file (inc/extras.php). For obvious reasons, I don’t want to change the extras.php file in the unite directory, I want to change it in my child theme.

I’ve tried creating the file (…/mychildtheme/inc/extras.php) but it doesn’t seem to be pulled in. Are these directories/files included recursively? Or is there something else I need to do to have Unite use inc/extras.php in my child theme directory?