The template you need to override sits on this path
woocommerce/templates/emails/admin-new-order.php
This template can be overridden by copying it to
yourtheme/woocommerce/emails/admin-new-order.php.
You can put your code there and this code will be executed for all the recipients that have been configured through the backend by going to
Woocommerce > Settings > Emails > New order > Recipient(s)
Hope this helps.