I don't understand what you want to do.
It is technically possible the download the numbers that represent code in a 328 chip but it is not practically possible to convert those numbers into C++ code that you could understand.
In any case, as far as I know, the code for the Arduino bootloader is included in the Arduino IDE so there is no need to download it from 328 chip.
On the other hand, if you are really wondering how to write an Arduino sketch on a PC and upload that sketch to a 328 that already has the bootloader installed see this tutorial.. You will need to have a working Arduino to do this or a USB-ttl cable - often called an FTDI cable.
...R