Updating an order with a new attribute

[findme]

I want to update received orders with a special tracking code. What is the best way to do this? Do I need to pre-define a custom field or can I simply add it on the fly? Are there pro's and con's to either approach?

[Brennan]

Where are you working with these orders? In the Miva Admin? If so creating a custom Order Field is going to be the best way.