Link entities to existing tables without code

Assume that we have java entities already implemented annotated with Jpa annotations. We also have an existed data base lightly different to the schema described by the entities above. how I can link the data base with my entities without the code. Otherwise, how can i proceed from the begin when implementin my entities to make this stuff configurable ( give to user the possiblity of specifying the names of columns corresponding to the fields of each entity in an externalized configuration file). NB: I use hibernate as an ORM.