How can I give the particular employee an additional amount, such as reimbursement, traveling allowance, or other pay?

To provide additional pay, you need to redirect to “Payroll ⇒ Assign pay component ⇒ Select the employee by entering the employee ID ⇒ Select pay Type ⇒ Select Variable ⇒ Select the Pay Period ⇒ Select the particular component and assign the value. 

Note: For the same component, the system will consider only the fixed component if there are values in both Fixed and variable.