Done: https://github.com/Fourdee/DietPi/issues/147#issuecomment-165469381
This will be available for v104 and new installations only.
If you want to move your swapfile on your current installation to USB, do the following:
You will need to change the swapfile size if required. Either in the code below, or dietpi-config > advanced after.
dphys-swapfile swapoff
rm /var/swap
echo -e "CONF_SWAPSIZE=100" > /etc/dphys-swapfile
echo -e "CONF_SWAPFILE=/mnt/usb_1/.swapfile" >> /etc/dphys-swapfile
dphys-swapfile setup
dphys-swapfile swapon