Commerce- How can I add a custom field to Account Information on checkout
I think you are looking for Commerce User Profile Pane module.


  Commerce User Profile Pane is a module that allows you to capture a
  Drupal Users profile fields directly in the Commerce checkout pages.
  This is similar to the Commerce Billing Profile or Commerce Shipping
  Profile panes, but instead of storing extra fields in the Commerce
  Profile entity, this will allow you to store it directly in the Drupal
  core User entity. This is useful when you want a quick registration
  process without the user having to fill a long user registration form,
  but you still want to retain the option to complete the User profile
  info in the checkout phase.