If u want add fields to folder, u should create mailItem fill it with
properties and move to folder, that's all.

That would be totally ineffective. Moving an item to a folder has no effect whatsoever on the fields defined in that folder.

To define a field in a folder in versions before Outlook 2007, use the MAPIFolder.Items.Add method to create a new item in the folder (a post item if it's a mail/post folder). Then use the item's UserProperties.Add method to create the property on the item, setting the third parameter to True. Finally, delete this new item, which is no longer needed.

--
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