I suggest keeping the imported number in one field where you can use it in reports and for searching. Use the auto-entered serial number as the key for any relationships.
The unanswered question that will complicate the original request:
What if the imported ID number matches a serial value generated in the serial number field?
If you can guarantee that such a match will never happen, (maybe the imported ID includes a letter, or you define the auto-entered serial number to include a letter), you can use a calculation that displays the imported ID if it exists and the serial number if it does not.
If ( IsEmpty ( ImportedID ) ; SerialNumberField ; ImportedID )
Thanks again PhilModJunk!!! You r the best! :)