If an employee is owed missed pay, is recurring Additional Pay the correct form to use?

Only if it is missed recurring additional pay.

If the employee is owed additional hours that have not been paid, it should be submitted via Payroll Request (ex: Off-Cycle) or through the I-181 time file.

Additional Pay transactions are used to pay an employee a regular flat dollar amount above their base compensation, on a recurring basis. For example, a $500 stipend paid once a month, or once a pay period. 

For scenarios in which the employee is owed missed or retro recurring additional pay, such as a retro stipend, then this would be the appropriate form to use.