Custom fields in the billing address section woocommerce

I have run into these issues before myself.
I am not entirely sure it’s possible to get adjust the billing address box directly unless you adjust the actual email template for the emails.

Specifically, if you have access to the file structure, look in the “/wp-content/plugins/woocommerce/templates/emails” folder of the WooCommerce Plugin. They are made to be modifiable in the sense that you should be able to use the wordpress theme structure to create a template by adding a new folder called woocommerce/emails and put the modified template in there.
There may be a way to do this within a plugin as well but thats a bit more advanced.

As far as making them editable check out this blog post: Mishas blog. Misha is awesome.

Also, here is a handy guide for the hooks in the emails I use that might be helpful.
Business Bloomer Email Hooks

tech