Você pode usar addScalar(String columnAlias, Type type) para declarar explicitamente o alias da coluna do seu SQL nativo:
getCurrentSession()
.createSQLQuery( "select e.id as id,e.first_name as firstName,e.password as password from xxxxxx")
.addScalar("id",StandardBasicTypes.INTEGER )
.addScalar("firstName",StandardBasicTypes.STRING )
.addScalar("password",StandardBasicTypes.STRING )
.setResultTransformer(Transformers.aliasToBean(Employee.class))
.list();