Answer
It's possible to set up various additional pay rates for employees in Lightning either individually or in groups.
To set up a pay rate to be applied to a single employee, go to Employees >> select the relevant employee from the list >> Pay Settings >> Special User Defined Pay Rates and add the rate in; this pay rate will then be available from the drop down list of pay rates in the Edit Pays screen.
To set up a pay rate that can be applied to a group of employees, go to Company >> Pay Rate Groups >> New Group and add in the pay rate, then Edit Employees to add said pay rate to a group of employees; this pay rate will then be available from the drop down list of pay rates in the Edit Pays screen for every employee you have selected.