ATMEGA328P-AU as USB Interface

No you can't. '328P does not have USB interface hardware.
I build MIKROE483 module into boards, FTDI FT232 based, like older Duemilanove had, but as a module is easier to solder.

Here's one that's in a socket on one of the boards I offer:
http://www.crossroadsfencing.com/BobuinoRev17/