How to conditionally link the expense type to employees.

Summary: I need your assistance with the client requirements as stated below.

I have created new mileage type "Mileage-Car Allowance" for those employees who receive a monthly car allowance. Now as per the client requirement, they want to associate this expense type to few employees, who are eligible for this car allowance.

Similarly, there is another expense type “Personal Car Mileage”. Client wants to associate this expense type to few employees, who do not have the Mileage-Car Allowance expense type attach to them. 

The client requires a way to associate these expense types with specific employees, allowing them to choose the expense type that is meant for them.