The need to link newly imported data in two different tables by an ID value supplied by a field defined in FileMaker in the parent table is a fairly common one.
This can be done pretty easily if you have a field or combinations of fields that uniquely identifies the parent record and also the set of related child records. The trick is to set up two relationships to two different occurrences of the Parent table. One is based solely on the identifier data imported into the table, the other is based on the auto-entered serial number field. You use the relationship based on the imported identifiers as a temporary link only for the purpose of copying over the serial number value in order to match values by serial number.
Say you have these table occurrences in these relationships:
Products|Import::Product = Features::Product AND
Products|IMport::Season = Features::Season
Products|SerialNumber::__pkProductID = Features::_fkProductID
Products|Import and Products|SerialNumb would be Tutorial: What are Table Occurrences?. __pkProductID would be an auto-entered serial number field. (Enable auto-enter options during import or use Replace field contents with the serial number option to assign a serial after the Products records have been imported.)
To get the needed value copied into _fkProductID for every features record, either enable an auto-enter calculation: Products|Import::__pkProductID or use Replace Field Contents to copy the value of this same Products|Import::__pkProductID field into the _fk field after importing the records.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
This was extremely helpful and solved my problem. Thank you so much!