code: private void addCustomFields(MpxjTreeNode parentNode, ProjectFile file)
     {
        for (CustomField field : file.getCustomFields())
        {
           final CustomField c = field;
           MpxjTreeNode childNode = new MpxjTreeNode(field)
           {
              @Override public String toString()
              {
                 FieldType type = c.getFieldType();
                 
                 return type == null ? "(unknown)" : type.getFieldTypeClass() + "." + type.toString();
              }
           };
           parentNode.add(childNode);
        }
     }
docs: Add custom fields to the tree.
  
  @param parentNode parent tree node
  @param file custom fields container