In the Cloud Active Directory Sync tool there are only two choices for what attribute to sync to DW for the user name: sAMAccountName and userPrincipalName and they are hard coded into the tool.

I suggest that the attribute be a fill in box that allows me to select the attribute that I want to sync much like the on premise one does. I have accounts that use the employeeID attribute to be their DW username so that a filter can be set up for them to only see their document when they log in.