If you're only using the default Pocket Outlook, I'm not sure there's a way to do it. In Pocket Informant 2005, I can hold the stylus over the appointment until the context menu appears, select "Tools", then "to task", and it will create a task based on the data from the appointment. It will also ask if I want to delete the original appointment.
Pocket Informant is a PPC PIM well worth its weight in gold. It's also very effective for GTD.