I have an empty webmail account and Outlook 2013 IMAP with non-root mail folders.
If I create the same non-root folder in webmail, it slurps up my Outlook messages. Some messages will remain, but I'm not able to get the other ones back on Outlook. It will sync new messages I place in the folders in Outlook. I have Outlook set to keep all offline messages, to not archive anything, and the greyed out default set to 99 months. Thanks for your help.
It looks like I have to move my inbox messages somewhere temporarily, move my messages in the Outlook non-root folder to Inbox, sync make the equivalent folder in webmail, then move the messages in webmail to the non-root folder. That's annoying.