Adding a plus one for this - add a new field to the ticket details form, and set it for agents only. You'll see when you configure the new field component that you can add some text to it (this appears on the tickets that users see, so be careful what you write!) and on the settings tab you can select only agents to view/edit, etc.
When a ticket comes in, the agent can add the hours to the field and when you export the tickets you are asked which fields to include, so include the new one in your list.
What we then need is a way to automatically calculate the hours and display it for the agents to see. In fact, I'd go further - an organisation could be allocated a specific number of hours (we sell support time in blocks of ten days, for example) and a ticket might take 4 hours to fulfil. If the agent entered 4 as the time allocated, then the result should be the allocation reducing by 4 hours. This should be visible on every ticket from the same organisation - providing the organisation has been created and the allocation added to it.
Any thoughts on how that could be done?