How do you add an expense account to the drop down box in the payroll deduction categories?

Hi peta2,

The categories available when trying to add a deduction on a pay run are based on what you've set up or added in Payroll Settings. If you would like to add a different deduction category to be used on a pay run, you can follow these steps.

1. Go to Employees tab.
2. Select Payroll Settings and then Deduction Categories.
3. Select Add and enter a Name for the Deduction.
4. Enter the Deduction Type and any other relevant fields, then select Save.

However, if you're trying to add an expense and be able to select a tax code on it you can create an expense category by following these steps. 

1. Go to Employees tab.
2. Select Payroll Settings and then Expense Categories. 
3. Select Add and enter the Name for the expense. 
4. Select Tax Code and then Save. 

Also, you can check on this article for your reference: Set up employee deductions. 

Let us know if you have any other questions. Have a nice day!