Is it permissible for an employee to purchase sunglasses and prescription glasses using the amount allocated for prescription glasses from the medical insurance, with an additional payment?

Commercial insurance is impermissible. However, this does not preclude benefiting from it if the agreement is between you and your employer, and not directly with the insurance company. In such a case, the sin falls upon the employer. As for the disbursement of the amount, it depends on the terms of the contract with the insurance company. If the agreement was solely for providing prescription eyeglasses, then you are not permitted to include sunglasses within that benefit.