Is it possible use a "normal" root partition e.g. from a USB HDD with Berryboot.
While it is possible to use a USB HDD with Berryboot, it will not have a normal root partition.
At least not until BTRFS gets more stable.
BTRFS supports subvolumes and could be used as an alternative to the current approach to have multiple operating systems sharing a partition.
Advantage would be that BTRFS is part of the main kernel, wouldn't require patches, and can easily be mounted from other Linux systems.
Maybe Berryboot is the wrong tool for this but I think this should be possible to do anyway.
Correct, it is not the right tool.
Berryboot's goal is making it easy for an average user to run multiple operating systems on the same storage. And to encourage users to try out other distributions, instead of just Debian.
It is not meant as a bootloader like grub/uboot.