External ID field on User Object

I am loading records for a custom object (Call Reports) into Salesforce, and want to assign records to specific users based on an external ID from another system. I have an external ID field setup on the User object, but when I go to map my fields in Scribe I cannot see this field on the Call Reports object. I can see the "OwnerID" field, but shouldn't my external ID field show up below that, since "OwnerID" is a lookup to User? I have refreshed my metadata several times, and tried logging in with Scribe Classic and the new UI.