Hi Janet,
We treat work time and overtime different, but, we can do the addition in attendance screen or in salary.
If you want the addition to show up in Attendance screen, we should use ‘Attendance Groups’ which will show the total under ‘Group Totals’
If you want the addition to happen in salary, just create another Non-payslip head and specify the formula.
Refer documents in the links below :
Salary-group
Formula-editor