KOrganizer supports the two dominant standards for storing and exchanging calendar data, vCalendar and iCalendar.
There are several tools to help you to import your calendar data e.g. from Microsoft Outlook® into KOrganizer:
Outlook2vCal is a tool for Microsoft Windows® to export data from Microsoft Outlook® as vCalendar file in order to import it to KDE's KOrganizer. Outlook2vCal is distributed under the GNU GPL and is available here for download on the Outlook2vCal project site.
A documentation on how to use Outlook2vCal is part of the KOrganizer documentation.