Mail.app in OS X — IMAP and Archive Mailbox subfolder creation
The only workaround I have found to add subfolders to the Archive mailbox is to add a new Mailbox (in Sidebar for the IMAP Account, Right click / New Mailbox…) Choose the account and for the name box enter:

Archive/"Name of the New Folder" (without quotes, remember the slash!)

This will create a new subfolder within Archive. :)




Anyone know any other way?