Import the iCalendar file to Outlook using Outlook's import and export wizard. 
This will prompt the user to select whether to import the event(s) to a new calendar or your existing calendar.

Outlook 2007:
1. Click File > Import and Export
2. Click Import an iCalendar or vCalendar file
3. Click Next
4. Select the iCalendar file and click Open

Outlook 2010:
1. Click File > Open > Import
2. Click Import an iCalendar or vCalendar file
3. Click Next
4. Select the iCalendar file and click OK


For more information please see this Microsoft Outlook support article.Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites, and we cannot assist with implementing resolutions from these websites.