Hi there, @BKCks.

Because mileage and pier diem are added in the Other Payroll Items section of the paychecks, we're unable to directly associate them to the job without any entries in the Earnings section.

However, you can follow these steps on how to add pier diem and mileage as non-taxable items in QuickBooks Desktop (QBDT). There's also an option to track these payroll items per job while setting them up.

Here's how:
 

1. Go to List and select Payroll Item List
2. Click Ctrl + New, then select Custom Setup.
3. Click Next and select Addition, then click Next.
4. Enter the name of the item and check the box for Track Expenses By Job box.
5. Click Next, then assign an expense account and select Next.
6. Choose None in the Tax Tracking Type drop-down and click Next twice.
7. Select between hours or quantity and then choose gross or net pay.
8. Enter an amount and select Finish.

Please take note that marking Track Expenses By Job won't add a Customer: Job column to the payroll items listed under Other Payroll Items on a paycheck. Instead, in reports, these items are divided based on the division of the earnings items on the paycheck. 

You can check out this article for information:  How to set up job costing and item tracking so you can track payroll expenses by job.

Don't hesitate to tag my name in the comment section below if you have other concerns or questions. I'm always ready to help.