The trick is to use different occurrences of the Customers table to define two relationships--one based on the name and the other based on the ID number. You can then use the name based relationship to copy over the correct Customer ID value.
If you don't know how to create duplicate table occurrences:
In Manage | Database | relationships, make a new table occurrence of Customers by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box as CustomersByName.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
Now you can create these relationships:
Jobs::FK = Customers::PK Customer ID
Jobs::Customer ID = CustomersByName::Customer ID
Duing Import, be sure to enable auto-enter options so that PK Job ID automatically assigns serial numbers to each imported Jobs record. (You could also have done this for your PK Customer ID)
Immediately after you import your Jobs records, your found set will be all 6,000 records. Click into the Jobs::FK field and use Replace Field Contents from the Records menu with this calculation to copy over the needed ID number from the Customers table:
CustomersByName::PK Customer ID
Dude, you are awesome. I knew it could be done... I didn't think of doing another table occurance. Well done!