There are two configuration files:

pom.xml
/src
  /main
    /resources
      hibernate.cfg.xml
      /META-INF
        persistence.xml

For pure Hibernate API, we should use hibernate.cfg.xml.
For JPA (Java Persistence API), the persistence.xml file should be used.

If we have a pure Hibernate API project, we can use the existing configuration file in a new JPA project.
Add in persistence.xml the property hibernate.ejb.cfgfile

<persistence 
   xmlns="http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
   version="1.0"
>
    <persistence-unit name="employee">
        <properties>
            <property name="hibernate.ejb.cfgfile" value="/hibernate.cfg.xml" />
        </properties>
    </persistence-unit>
</persistence>