Answer
To add a pay click on Create Pays, select the employee(s) whom you wish to record a pay for within that period. Then select the date you would like this pay to be processed or if it were a past pay, the date it was processed.
The tickbox at the bottom of the window which says Create Full Pay if ticked will filter the information from the Pay Settings for each employee.
If the additional pay is for a bonus or other irregular payment please see here for an example on paying a bonus, commision, etc.
If the additional pay is for a back payment please see here for an example of making a back payment.
If you need to delete an additional pay row from the pay run, please see here to learn how to do this.