Nick here from Shopify.
It's definitely possible to add your EIN number. This would need to be sent to our tax team to review though. I have reached out to you directly via email to verify your store details and once you reply to me there and confirm them, I can send it to that team for you. If for any reason you didn't get an email, please let me know here.
Great question! We can't look into account specific requests via the community right now for privacy and security reasons. Your best next step here is to contact someone on our support team about this if you want to talk to someone on the tax. You can contact the support team directly here to log in and let the support advisor know what you are looking to do and they will point you in the right direction to the tax team!
If you have any other questions about any other aspects of your store, don't hesitate to let me know!