You can do this a few different ways. It depends on what functionality you want down the road.
Option 1:
(this will only work if on your "Jobs" table the key field is the unique job number you mentioned)
Make the [Related Job] field that is part of your relationship a "Formula-Text" field and make the formula use the unique value from the API import.
Pros: The child records will be automatically associated with the Job.
Cons: You can never change that back, and have all the associations stay. This will also affect all connections retroactively.
Option 2:
Make that [Related Job] reference field be conditional. Where that unique number is the driving factor.
Pros: You can have always turn off that conditional restriction if things change down the road. But at least it only one item in the drop-down menu.
Cons: You have to manually select the related job.
If you have any other details about this, we could possibly work a different angle.