Can I make a directory on OS X appear as a volume so I can backup using Super Duper?

I use Lightroom and keep my library on an external hard drive. After I quit Lightroom I use SuperDuper! to back up that external hard drive to an identical external hard drive. I also have two Transporters , one on-site and another off-site. I would like to backup to the on-site Transporter. The Transporter Library shows up as a folder. I would like to create a sub-folder and then use SuperDuper! to backup to that folder. However SuperDuper only allows you to backup to a mounted volume. Is there a way on OS X to make a directory appear as a volume so it would appear in the to: drop down of SuperDuper! or would the recommended way to be backup to a Sparse Bundle Disk Image located in the Transporter folder?