You may need to allocate overhead expenses to your customer jobs in addition to those expenses incurred directly for a particular customer job. This article provides a basic method of allocating overhead costs.
Note: There is a more advanced method which includes using items.
Create an account and journal entry for tracking:
Since the net effect on the allocated expenses account is zero, it should not show on the Profit & Loss report.
Create the Job Profitability Detail report for one of the customer:jobs and you will see the allocated overhead expenses at the bottom of the report in the "No Item" row.