I had always understood that employers could not require you to participate in a contributory plan, but I have to confess that I cannot provide you with any law that says so. Will your employer allow you to opt out if you can provide proof of other coverage? That would definitely hold water legally if they will.
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.