Child Theme – Needing to change admin functions…

Hi there,
I’m using the Nash theme (from ThemeForest) for a site under development. I’ve added a couple of post types that need to also be added into the backend theme options area. I’m trying to do these sorts of additions/changes using a child theme, but I’m not sure how to do this. It requires making additions to a file that would be located here: wp-content > themes > nash > admin > functions > functions.options.php

And obviously I’d rather my additions ONLY reside at wp-content > themes > child-theme > admin > functions > functions.options.php

Is there any way to do this? Or do I need to resign myself to not being able to use updates to this theme as they might come out?

Thanks in advance.

~Laura