Yes it's very easy.
Boot it with your new formatted usb plugged in.
Go to system setting, storage device, and change to your usb drive. It'll reboot.
After reboot, Shut it down, and plug your drive into the computer. You will find a new folder called "recalbox". it will have all the sub directories on your recalbox. Roms, Bios, etc..etc...
Copy your ROMS and Bios over the network from your recalbox (SDcard) to your USB (on your laptop) and then go put your USB in the pi and boot up.
Double check the config, that storage is pointng to the USB and you're all set.