For your custom field, have you thought about using a drop-down list that is populated by values from the 'Login' column in the [FNMSCompliance].[ComplianceOperator] table?
Creating an additional custom field as a Drop-Down List (UIFieldTypeID=8) is documented in the FNMS "System Reference" document. The more interesting part is updating the options to be displayed when data in the [ComplianceOperator] table change.
It's a possibility. The problem is the dropdown list would need to be populated with thousands of names. Keeping it up to date as people come and go will be a bit if a challenge as well. I've got an idea which I was going to try for another attribute. If that works out, I could use it here. It would simplify things if FNMS had a lookup attribute that would pull from an existing table - such as the User table.
I think that attempting to create a Custom Field and maintaining it with a list of every UserID is not something that can realistically be maintained and would not be scalable.
I think that at present, the best you can do is to create a custom 'Audit Report' that lists all of the current values entered into this Custom Field and then compare this to the list of active Users. For FNMS On-Premises, this would be possible in SQL, and for FNMS Cloud would be a bit more of a manual process.
