title: woocommerce - custom field in order
body: <p>I have the need to have a custom field in the admin 'edit order' screen of woocommerce.</p>
  
  <p>This field is admin only and does not need to display on the customer order process.</p>
  
  <p>The business case is that the person processing the orders clicks in to the edit order screen for each order and assigns a custom ID (these are random serial numbers on our products) from a drop down list preferably.
  The drop down list will be populated from a database of unassigned ID's - (this is a sql database we already use for our main web app) </p>
  
  <p>My question is, is a custom field in the 'edit order' screen the place to perform this assignment? 
  the alternative is to write a small .net application and assign the Id's there but I would like to keep the order processing to just woocommerce.</p>
  
  <p>If it is possible what hooks should i look in to.</p>
  
  <p>thank you</p>