No. Therefor you have to use debootstrap I think. With usb root you can make a firmware with all packages you want and copy it onto a usb stick. To be more precisely you have to copy build/modified/filesystem onto your stick. Don't forget to flash a "small" image with usbroot into your flash. So that you can activate usb root with "/etc/init.d/rc.usbroot on" and "/etc/init.d/rc.usbroot store /dev/sdx:/filesystem".