Extended Properties using WEBDAV Exchange 2003

Hi,

Our company is going to be migrating from Exchange 2003 to 2010 soon. As part of this migration I need to rewrite a number of client apps written in c# that use WebDev to update/create appointments in Exchange, and get them to use EWS instead.

I have a c# console app that is run overnight and updates a bunch of appointments in Exchange based on data that is contained in a SQL database. Previously the ID of the record in the database was used to create/update the appointments using the URL 

http://mymailserver/exchange/username/calendar/12345.em

where 12345 would be the ID from the database.

I want to move away from doing this to using extended properties once we have migrated to 2010 and updated the c# app to use EWS.

In order to facilitate this process I need to be able to create an extended property called VisitID where I will stored the ID from the database in.

I have managed to do this using EWS but I need to this using WEBDAV, as I need to update all existing appointments that are on Exchange 2003, before the migration occurs.

My original app uses this as a basis for adding/updating appointments : http://msdn.microsoft.com/library/office/ms877306(v=exchg.65).aspx

Has anyone got sample code of adding an extended property using a WEBDAV call as that would really help me!