procedure TForm1.AddFilesToList(Filter, Folder: string; Recurse: Boolean);
var
  sr: TSearchRec;
  sDirList: TStringList;
  i: Integer;
begin
  if FindFirst (Folder + Filter, faAnyFile, sr) = 0 then
  repeat
    ListBox1.Items.Add (Folder + sr.Name);
  until FindNext(sr) <> 0;
  FindClose(sr);
  if Recurse then
  begin
    sDirList := TStringList.Create;
    try
      GetSubDirs (Folder, sDirList);
      for i := 0 to sDirList.Count - 1 do
        if (sDirList[i] <> '.') and (sDirList[i] <> '..') then
        begin
          Update;
          ListBox1.Refresh;
          AddFilesToList (Filter,
            IncludeTrailingPathDelimiter (Folder + sDirList[i]),
            Recurse); // hier soll geändert werden
        end;
    finally
      sDirList.Free;
    end;
  end;
end;

procedure TForm1.btnSourceClick(Sender: TObject);
var
  Dir: string;
begin
  Dir := CurrentDir;
  if SelectDirectory ('Choose Folder', '', Dir) then
  begin
    CurrentDir := Dir;
    AddDelphiFiles;
  end;
end;