Hello,
etheme_catalog_setup function is located in the xstore/framework/woo.php (line 747) and you are able to change it using child theme and it will work.
ThemeForest has a number of requirements to submit theme for selling, We have created XStore Core plugin only because of the ThemeForest requirements and moved all the functions and options that couldn’t be placed directly in theme according to these requirements.
What function from the XStore core plugin do you want to redeclare? Tell us and maybe we’ll find some workaround for you.
Regards