Oh, I probably misunderstood your problem. I thought you messed up your phone and you want to restore stock rom to fix it.
If you want to flash another rom, just run "Clean to install a new ROM" option and then install the zip.
If there isn't a ROM installed, you can't access to internal storage via USB cable in fastboot or recovery mode.
I usually use a pendrive in this situation and I flash the zip with OTG cable from pendrive.
Thanks for valuable help.