No, that's not possible. iCalendar doesn't support it.

Signature
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I need to be able to change the field "show time as" to out-of-office or
> tentative within a vcalendar/icalendar file.
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>
> Dimitri