Magento version 1.14.x
Problem:
Create a custom order attribute and display it on the admin new order form (and view order). Display the attribute in an existing fieldset/box.
Solution:
Create custom extension. However, consider not displaying the attribute in an existing fieldset to avoid template override (display in separate fieldset/box instead) as explained here: http://www.webspeaks.in/2015/12/magento-add-custom-block-in-admin-sales-order-page.html.
Use code below as reference only. Code is incomplete.