A User Manager field exists that is inaccessible from the User Manager GUI—Usercomment. Similar to the GUI-accessible Description and Comment fields, Usercomment is 48 characters long, and you can use a net user /usercomment: command to populate it.
In our environment, we needed a place to put employee ID numbers, but because of their sensitive nature, we didn't want these ID numbers to be visible. We use the following simple batch file to populate this field and ensure that each user has an associated employee ID number:
net user %1 /usercomment:"%2"
I haven't found a third-party utility to parse this field, so I use the built-in Net User command.