No, it is not possible to set free/busy information at all. F/b info is read-only.

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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
> Im trying to know if it's possible to programmatically set free/busy
> infromation of an attendee right after his address is entered to the
> appointment. I would like to use that for invitees with "no
> information" (hatched) and to add the free\busy information I have at
> that moment.