how to add dirs to live debian

How would I add a directory such as /root/mydir to a live debian image I've created by using 'lb config' and 'lb build'?

Just create the directories you would like to add to the Live image in your config/includes.chroot/ directory.
Example:

mkdir -p config/includes.chroot/root/mydir

will create a directory that will be /root/mydir in the image.
More info on this in the Debian Live Manual in the customizing-contents chapter.

Thanks