You can process approved Expenses into the Payroll system by mapping each category to the respective pay elements in Payroll.
NOTE: Please make sure the pay elements you want to use are already created in Payroll. See Payroll > Configuration > Payroll > Pay Elements.
Step 1: On the Categories page, put a tick into the option - Transfer to Payroll.
Step 2: Click on the 
icon under the Actions column.
Step 3: A pop-up window will appear. Select the pay element that you want to map the category to.
Step 4: Click on 
to save the setting. Repeat steps 2-4 for each Expense category.
Next step: Reimburse Expense
Final step: Process Expense via Payroll