@Link You can create a folder for the ROMs on the /media/usb0 partition, copy the contents of /home/pi/RetroPie/roms to it, then symlink /home/pi/RetroPie/roms to the folder on the HDD. Make sure you create a back of /home/pi/RetroPie/roms first, just in case.
Or you can bind mount the folder over /home/pie/RetroPie/roms with something like
mount -o bind /media/usb0/<Folder_name> /home/pi/RetroPie/roms