Mileage is a nontaxable item so you will not want to add this to their taxable wages.
Mileage reimbursement is not considered taxable compensation so you'll want to enter the amount as an Employer Reimbursement for payroll! You can use the current federal mileage reimbursement rate.
If the amount varies per pay period, you may enter the amount each pay period in the Next Payroll tab. If the amount will always be the same, you may set it as a Default Setting.
To do this, go to Settings -->Employee Info, then click Edit next to Pay Settings. From here, you can add the default amount under Employer Reimbursement. Make the updates needed and then click Save & Reset Payroll. Then you will be directed to the Next Payroll tab to review your upcoming payroll.