You can only create subfolders if you've opened the user's mailbox through the properties of the Exchange service, in other words if you can see it in the folder list. Other users would have to do the same to see those subfolders:

To access a shared folder that isn't one of the folders listed on the File | Open | Other User's Folder dialog, the mailbox owner needs to grant "folder visible" permission to the root of the mailbox and any other parent folders of the shared folder, as well as appropriate permission -- at least Reviewer -- on the shared folder itself.

The user who needs access then goes into Tools | E-mail Accounts or Tools | Services (depending on the Outlook version), brings up the properties for the Exchange Server service, and on the Advanced tab, adds the mailbox.

For more information on folder permissions, with how-to screen shots, see http://www.howto-outlook.com/howto/permissions.htm and http://office.microsoft.com/assistance/preview.aspx?AssetID=HA011134811033