Hi Simon,
this is not possible within the UI at the moment. You will need to edit the source code to achieve your goal.
I recommend you to have a look at the customization section of the forum under 1.9.x, but also under the 1.8.x part of the forum - there are several threads regarding "adding a column to the ticket queue pages".
Beside that let me mention that the developers already plan to add a feature like this, called "custom queues" in a future release of osTicket (no ETA).