Okay - here's the trick to submit data to that type.
Your data needs to look like:
-1;#JDoe
That is, a minus1, a semicolon, a pound sign, then the user account id.
So, you may want to add another field for this, and map the new field instead - then either use rules or a default value to set the field.
Or, of course, you can change your SharePoint field type to just be text.