C#

class BindingWithCustomColumns<T> : ICustomTypeDescriptor {
 T obj;
 Dictionary<string, string> CustomValues { get; private set; }

 public BindingWithCustomColumns(T t) { obj = t; CustomValues = new Dictionary<string,string>();}
 
 // bunch of methods that need implementing to do nothing

 public PropertyDescriptorCollection GetProperties() {
  PropertyDescriptorCollection props = TypeDescriptor.GetProperties(obj);
  foreach(string key in CustomValues.Keys)
   props.Add(new CustomPropertyDescriptor<T>(this, key));
 }

 class CustomPropertyDescriptor<T> : PropertyDescriptor {
  BindingWithCustomColumns<T> host;
  string key;
 
  internal CustomPropertyDescriptor(BindingWithCustomColumns<T> host, string key){
   this.host = host; this.key = key;
  }

  // some more stuff you need to implement

  public object GetValue(object component) { return host.CustomValues[key]; }
  public void SetValue(object component, object value) { host.CustomValues[key] = (string)value; }
  public Type PropertyType { get { return typeof(string); } }
  public Type ComponentType { get { return typeof(BindingWithCustomColumns<T>); } }
 }
}