Is it permissible for an employee to subscribe to health insurance whose premiums are borne by the company, knowing that if he does not wish to do so, he will be deprived of receiving the costs of treatment that the company previously covered?

If an employee is entitled to medical treatment from their employer, and the employer decides to refer them to an insurance company to fulfill their right, then there is no blame upon the employee for that. The sin in commercial insurance falls upon the employer, not the employee.