<hibernate-mapping>
  <class name="Contact" table="contacts"> 
   <id column="id" name="id">
      <generator class="native"/>
   </id>
   <property name="name" column="name" type="string"/>
   <dynamic-component name="customProperties">      
   </dynamic-component>     
  </class>
</hibernate-mapping>

public class HibTest {
    private static final String TEST_FIELD_NAME = "s1";
    private static final String TEST_VALUE = "test@test.com";
    public static String CUSTOM_COMPONENT_NAME = "customProperties";
    private Class entityClass = Contact.class;
   
   
   public HibTest() {
      Configuration configuration = new Configuration().configure();
      SessionFactory sessionFactory = configuration.buildSessionFactory();
      Session session = sessionFactory.openSession();
        configuration.buildSessionFactory().openSession();     
       
        Column column = new Column(TEST_FIELD_NAME);
        column.setName(TEST_FIELD_NAME);
        column.setNullable(false);
        column.setUnique(false);
       
        PersistentClass persistentClass = configuration.getClassMapping(entityClass.getName());
        persistentClass.getTable().addColumn(column);
        SimpleValue simpleValue = new SimpleValue();
        column.setValue(simpleValue);
        simpleValue.addColumn(column);
        simpleValue.setTypeName("string");
       
        simpleValue.setTable(persistentClass.getTable());
        Property property = new Property();
        property.setName(TEST_FIELD_NAME);
        property.setValue(simpleValue);
        property.setNodeName(property.getName());
        Component customProperties = (Component) persistentClass.getProperty(CUSTOM_COMPONENT_NAME).getValue();
        customProperties.addProperty(property);
       
        session = configuration.buildSessionFactory().openSession();
        try {
           Contact contact = (Contact) session.get(Contact.class, 4);
            Object value = contact.getValueOfCustomField(TEST_FIELD_NAME);
            System.out.println("value = " + value);
        } catch (Exception e) {
           System.out.println("e = " + e);
        }
   }
   
    public static void main(String[] args) {
       HibTest test = new HibTest();
    }
}

public class Contact {
   private int id;
   
   private String name;

    private Map customProperties;
    public Map getCustomProperties() {
           return customProperties;
    }
    public void setCustomProperties(Map customProperties) {
           this.customProperties = customProperties;
    }
    public Object getValueOfCustomField(String name) {
        return getCustomProperties().get(name);
    }
    public void setValueOfCustomField(String name, Object value) {
        getCustomProperties().put(name, value);
    }
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
}

Exception in thread "main" org.hibernate.MappingException: property mapping has wrong number of columns: Contact.customProperties type: component[]
   at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:441)
   at org.hibernate.mapping.RootClass.validate(RootClass.java:192)
   at org.hibernate.cfg.Configuration.validate(Configuration.java:1102)
   at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1287)
   at HibTest.<init>(HibTest.java:46)
   at HibTest.main(HibTest.java:57)

XmlDocument xmldoc = new XmlDocument();         
         xmldoc.Load("Model/Driver.hbm.xml");
         XmlNamespaceManager nsMgr = new XmlNamespaceManager(xmldoc.NameTable);
         nsMgr.AddNamespace("nh", "urn:nhibernate-mapping-2.2");
         XmlNode node = xmldoc.SelectSingleNode("/nh:hibernate-mapping/nh:class/nh:dynamic-component",nsMgr);         
         
         XmlElement element = xmldoc.CreateElement("property");         
         
         //Create new attribute Name
         XmlAttribute attrName = xmldoc.CreateAttribute("name");
         attrName.Value = "NewField";

         //Create new attribute Type
         XmlAttribute attrType = xmldoc.CreateAttribute("type");
         attrType.Value = "string";

         //Add the attributes to the property node
         element.Attributes.Append(attrName);
         element.Attributes.Append(attrType);
         
         //Append the propperty node
         node.AppendChild(element);   
      
Configuration.AddDocument(XmlDoc)