USB interface on arduino severino

hi to all
i've a couple of arduino severino with me on which i'm working for a while.
it have a serial interface for programming and other application

as arduino seems to have seemless practicality so i'm trying a serial to USB interface IC on it.

i've purchased a PIC 18F13K50 chip which converts Serial interface to USB interface. as it is a low power device, i want to power it by arduino board and program by Arduino IDE

so any one out there have tried anything like that, or have any idea how will it work? please help me out

waiting for a reply
thanks