Easy. View -> Preferences -> Database tab. Highlight Format in the list in left-hand box. Click Edit Selected List. Type in whatever you want to add and click OK. You can move your entry up (or down, later) using the arrows just above Remove. And you might want to shorten the list using Remove if there are formats you never use.

You can edit any of the other lists the same way and, of course, can add custom fields and select your favourite character set from the same tab.