you are right, no one incl. Admins have permissions on this new attribute by default.
For giving permissions to the Portal Admins to modify this attribute, you have to add the new attribute to the attributelist in the Set:
Administration: Administrators can read and update Users
In addition to that, if you want the SyncService to export data for that attribute to the FIMService you need to add this attribute also to the Set:
Synchronization: Synchronization account controls users it synchronizes
In my environment I set this to All Attributes for Admins and SyncService in order to make adding attributes to objecttype easier.
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