This falls into two steps: firstly exporting an Outlook calendar as .ical files and secondly turning those .ical files into a .ics file.
The technique was used with Outlook 2003 running on Windows XP (as part of a migration to Xfce's calendar, Orage). It probably works with other versions and OSes too.
Thanks and acknowledgements to BlueDevilFan
who posted the original VBA macro at Experts Exchange
where it is no longer visible (but may be...