procedure TForm1.Button1Click(Sender: TObject);
var
  BMP: TBitmap;
  PNG: TPngImage;
begin
  PNG := TPngImage.Create;
  try
    // load PNG from somewhere
    BMP := TBitmap.Create;
    try
      BMP.Assign(PNG);
      // here's your transparent bitmap
    finally
      BMP.Free;
    end;
  finally
    PNG.Free;
  end;
end;