If you have additional information you'd like listed on your billing documents for all your clients you can add this info in the footer for statements, superbills and invoices.
In SimplePractice go to My Account > Settings > Documents > there will be a separate section for each billing document and a text box for your footer information. Enter your info in the text box and click save. This won't update documents that are already created. Create a new billing document of that type and it will have your footer at the bottom of the document.