How can I add fields to a clientdataset at runtime?

Question

I have a TClientDataSet, which is provided by a TTable’s dataset. The dataset has two fields: postalcode (string, 5) and street (string, 20)

At runtime I want to display a third field (string, 20). The routine of this field is getting the postalcode as a parameter and gives back the city belongs to this postalcode.

The problem is only about adding a calculated field to the already existing ones. Filling the data itself is not the problem.

I tried:

  cds.SetProvider(Table1);
  cds.FieldDefs.Add('city', ftString, 20);

  cds.Open;

  cds.Edit;
  cds.FieldByName('city').AsString := 'Test';  // --> errormessage (field not found)
  cds.Post;

cds is my clientdataset, Table1 is a paradox Table, but the problem is the same with other databases.

Thanks in advance