Not sure if I understand your question, but if you take a drug whose patent is expired which may be available as a generic, and you combine that drug with one that is still patented, the combo drug gets a whole new patent.

Of course this doesn’t mean that someone can’t write a prescription for each of the 2 drugs, assuming that the patented drug and the new drug are both available alone and not just in the combo, but most doctors aren’t going to do that.